Can You Run an Art Gallery?

Did you have your Sim buy the retail lot? Did your Sim set them for sale? Do you have a cash register on the lot? All those things are basic needs to be done before you can open the store and sell paintings. I could go into detail, but @Writin_Reg gave a very good suggestion to check Carl's Sims 4 guide. There's a wealth of information there on all aspects of the game, I go there all the time. Here's a link if needed:

Carl's Sims 4 Game Guide for PC, Xbox and PS4