the frame if it didn't ship with disc brakes , the tab on the frame may not have been faced , the surface, machined flat and in alignment.

but the bike shop should be able to do that, if required..


or, of course consider taking it in for a a trade-in and getting another bike even if the same one the bump up to disc brakes , as stock.

the shop you got it from in the 1st place should make you a fair offer.