Is it a mitzvah? Rambam and Ramban disagree.

Everybody agrees that there are 613 commandments; however, different sages count certain verses in the Torah as mitzvot while others versus are not considered mitzvot. We shouldn’t be surprised that Rambam in his introduction to his Sefer HaMitzvot enumerates 14 principles (Shorashim)  which determine what counts as a mitzvah. The third shoresh is “You should know that that which they said (Makkot 23b), "613 commandments were stated to Moshe at Sinai," indicates that this is the number of the commandments that are practiced for [all] generations. For commandments that are not practiced for [all] generations do not have a connection to Sinai - whether they were stated at Sinai or elsewhere. ” (Sefaria.org translation)

Daf TB Khullin 24 sets up a debate between Rambam and Ramban whether carrying the tabernacle and its vessels is a mitzvah are not. According to the Torah the Levites only serve between the ages of 30 and 50. The Gemara expands their opportunity to serve after a permanent home in Israel is built and the tabernacle no longer needs to be carried from place to place. “One might have thought that the Levites were disqualified with the passage of years even in Shiloh, the permanent place of the Tabernacle, and in the eternal Temple. Therefore, the verse states: “To perform the work of service, and the work of bearing burdens” (Numbers 4:47), juxtaposing the two forms of Levite service to teach: I stated the disqualification of the passage of years only at a time when there is Levite service involving carrying the Tabernacle on their shoulders.” (Sefaria.org translation) Since carrying the Tabernacle is not something that is practice for all generations, Rambam doesn’t count it as one of the 613 mitzvot.

Ramban agrees with the principal; however, he disagrees on its application. Carrying is counted as one of the 613 mitzvot. He explains his point of view in his commentary on Numbers 8:25.

“AND FROM THE AGE OF FIFTY YEARS HE SHALL RETURN FROM THE SERVICE OF THE WORK, AND HE SHALL SERVE NO MORE — “the service of carrying loads upon the shoulders, but he returns [i.e., he may still serve] to close the gates, or to sing, or to load the wagons.” This is Rashi’s language. But in the Sifre the Rabbis mentioned only that “he returns to close the gates or to the work of the sons of Gershon” [but they did not say that he may return to the service of singing]! And this indeed appears correct [that a Levite above the age of fifty was not allowed to take part in the singing] — for if it were as the Rabbi [Rashi] has written, why were the twenty-year old Levites not appointed to sing, to close the gates and to load the wagons until they reached the required age, and why were they counted from thirty to fifty years old and only for the work of bearing burdens [since both before and after these two age-limits they could still serve by singing etc., therefore they should have been counted from twenty years, with no upper limit]? Moreover, why were the sons of Gershon and Merari counted in this way [from thirty to fifty] since all their work [i.e., singing, closing the gates and loading the wagons] could be done when they were older?

"We must rather say that since they were appointed to carry the ark at the age from thirty to fifty, they [therefore] were not appointed to sing, which was the main function of the Levites, unless they were also qualified to carry the ark, for all those who were appointed to sing were qualified to perform all the services [including the carrying of the ark]. And since the Kohathites [who were the only ones permitted to carry the ark] were counted from thirty to fifty years old, even for singing, they were all counted in this manner, in order that the sons of Gershon and Merari should not be qualified to sing during these years [i.e., below the age of thirty and above the age of fifty] and the sons of Kohath be disqualified. [Hence the age limits of thirty and fifty applied to the Kohathites’ singing, which was their main function, as well as to their carrying of the parts of the Tabernacle, and therefore these age limits applied to the function of singing of the sons of Gershon and Merari as well, and they could not, as Rashi wrote, return to the singing.] But for closing the gates or loading the wagons all of them were qualified [even after the age of fifty, as quoted in the Sifre above]. Furthermore it is written, from thirty years old and upward even unto fifty years old, every one that entered in to do the work of the service, and the work of bearing burdens in the Tent of Meeting, and the Rabbis interpreted in the Tractate of Shechitath Chullin: “I might think that in Shiloh and in the Eternal House [at Jerusalem, when the services of the Levites were limited to gatekeeping and singing] it was also so [that the Levites were disqualified after the age of fifty]; Scripture therefore says, to do the work of service, and the work of bearing burdens. I have only said so with respect to a time when there is bearing burdens upon the shoulders.” Now [the expression] ‘avodath avodah’ [the work of service, but literally “a service of a service”] is a reference to singing, as Rashi has written above in Seder Naso; if so [it is clear that] when there was [the duty of] bearing the burdens upon the shoulders, the Levites were disqualified from singing as well [after the age of fifty]. And so it is said in the words of David, And the Levites were numbered from thirty years old and upward, and it is further written there, Of these twenty and four thousand were to oversee the work of the House of the Eternal etc., and four thousand praised the Eternal with the instruments which I made to praise therewith. For until the House [of G-d in Jerusalem] was built, when they still had to carry [the ark and the Tabernacle] on their shoulders, they only appointed Levites to sing who were also fit to carry [i.e., from the age of thirty to fifty]; but David counted them again from twenty years old to meet the requirements of the House [of G-d] when it was built, as has been explained [above at the end of Verse 4].” (Sefaria.org translation)