@jasleighxo I'm wondering if the lot you bought was a retail store not a Vet Clinic? A Vet Clinic does not use a cash register. (Unless you are referring to the cash register on your sim's phone.)
Please go to the 'Lot Type' in build/edit mode and change the lot type to Vet clinic, then make sure that you have all of the required items on the lot. There is some helpful information at this link:
https://www.carls-sims-4-guide.com/expansionpacks/pets/veterinarian-clinic/
If you place a Vet Clinic in your world it should run automatically by the game (if set up properly) on it's own, and if you have a Vet sim who purchases said clinic it should still be run automatically by the game... plus when they are select 'open' (like when they own a restaurant) the employees will come and when they go to the clinic and it is open they will be the vet (along with any other employees they have).
Could you please post a screenshot of what your phone interface looks like for this? Thank you!
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