Do you have shared users on one or both locations?
Every Samsung account in SmartThings is supposed to have a “default location”. Some users end up with two default locations due to account sharing. Here’s an example: User A is the owner of Location A which is a default location. User B is the owner of Location B which is also a default location. User A shares access to Location A with User B. User B will now see two default locations.
Where it can get even messier is with account migration. User A is a SmartThings account and the owner of Location A. Location A is shared with User B which is a Samsung account. Because User B is a Samsung account and User A is not, user B is considered the owner on the Samsung side because there isn’t a record of User A there. User A then migrates the SmartThings account to a Samsung account. If the Samsung account for User A was previously used, there may already be a default location for that user, Location C. So again, we end up with two default locations.
It is possible to untangle things so that User A is the correct owner of Location A which will then allow for the deletion of Location C. BUT in doing so, User B has to get their own default location, we’ll call it Location D. This results in User A only seeing Location A, but User B sees Location A and Location D, neither of which User B can delete.