Why can't I place a Tool Cupboard?
A Tool Cupboard can only be placed if all of these are true:
You have building privilege on the spot — meaning either no other TC controls this area, or you're authorized by it
The structural element you're standing on is part of a building that doesn't already have a TC
The placement spot isn't blocked by terrain or other structures
Why building privilege matters: every TC creates a build-zone around the structural elements of its building. Inside that zone, only people authorized by the TC can place new structures or another TC.
How to check: Look for the "Building Blocked" or "Authorization required" message when you try to place. If you see it, you're inside someone else's TC zone.
If none of the above applies and you still can't place a TC, please submit a Bug Report.
→ See also: Why did my base split into multiple buildings? · How do building privileges work?