CiCiinSimsCity This is definitely a mod problem, as in the vanilla game there is no option to use the cash register to check out and sims don’t queue there. They just get the checkout symbol above their head and you have to send your sim over to them to check them out.
Try removing your mods folder entirely (drag-and-drop to desktop or rename) and also delete the cache file. Then reopen and test in a new save. There should not be any option to check out at the cash register. See if your sim can complete the checkout action once that option no longer appears.