Sim can't go upstairs in own restaurant

@Editingbynikki Thank you fro sharing your lot for testing! Your restaurant is interesting and attractive. Unfortunately, I had a number of issues in testing it. A few of the things I suspect may be causing you issues are:

• I suspect the heavy use of platforms, plus having many of them sharing the outer wall, could be part of the issue.
• Your patio area is made of a combination of curved/straight areas/fencing that seemed to be combined with a (rounded) platforms (perhaps overlapping?), as when I changed the foundation to a plain one, you can see that there is still a foundation trim that is not part of the fencing design:
  With foundation changed:
  
  This is what I see if I choose a lighter color of platform trim:
  
  I happened to notice this same thing upstairs in the 'room/area' where the piano is:
  With the old platform trim:
  
  With the new platform trim (lighter color):
  
• I also could not get the Sim to enter the building or go anywhere in it. After I changed the foundation and highlighted the room with the stairway and created the ceiling in the area, I was able to direct my Sim to go upstairs:
  
  Unfortunately, though, even then she could not go back downstairs.
• As an example of the mixed regular and foundation rooms/areas...
  As you can see in this picture from inside, on the main floor (with room/s above it), with the foundation changed to a plain foundation you can see that these rooms are a mix of normal wall/room (area that the new foundation applied to) and a room(s) created with platforms:
  
• Another unusual thing, when I was testing some things on the main floor, I tried deleting this section of wall. I literally deleted (MOO not active) the wall with the 'delete wall' tool three times before the wall actually deleted. And each time there was monty credited and the animation/sound to indicate there was deletion. I'm assuming this may indicate that there were layers on top of each other, as with MOO not enabled using that wall deletion tool should have deleted everything on the wall and the wall itself.
  
• As to your comment "When I click an upstairs chair and want to select 'sit' it says 'Alan can not sit at a table that is not his'", this is actually normal when you own a restaurant. An owner Sim cannot sit at any table you ad to the lot, as tables added on the lot are considered as part of the seating available for customers only. A workaround I have done on my Sims' owned restaurants is to add a refrigerator and stove or extra cooking station (that is not occupied by a cook - otherwise you have to do this while the restaurant is closed) and bar with stools (bar seating is not considered to be customer-only seating, even if you hire a bartender when open... then the owner Sims can make a dish here and there and put it in the fridge and they can then grab a plate and sit at the bar to eat.
• As to your restaurants and vet clinics not showing in the panel as owned by your Sims, did you check if this issue showed when your Mods folder and cache file were removed? That may be an entirely different thing than the interaction issues on the restaurant lot.
  You could also try a game repair, though I think that would be more a 'Hail Mary Pass', but it wouldn't hurt:
  
  Spoiler

  How to run a game repair:
  
  • Open Origin, click ‘Game Library’ then right-click Sims 4 > Repair Game
  • Let it run until it has completed all repairs. It will do the base game first, then run through each of your DLCs (Expansion Packs, Game Packs, Stuff Packs and Kits).

I'm sorry that this is not a solution, but it may give you an idea of some of the issues to troubleshoot on the lot. I know there are some different parameters with platforms, and they do not work the same as a room. Some of the nuances of this may be important to research and troubleshoot also. Please let me know if I can be of further assistance.