If you’ve encountered anything like this problem, then you may have had to update the permissions that a specific SQL login has. However, if the user you’re trying to edit is mapped to dbo, then you’re not allowed to make changes.
There are ways to do it, but you need a pretty thorough understanding of what you’re trying to achieve.
There’s a quick way around this using only the GUI. In SSMS, open the properties of the database(s) you need access to. In the ‘Files’ page, you’ll see the Owner of the database. This is quite likely the account you want to add additional permissions to. But since it’s mapped to dbo, you can’t. Therefore you update the owner of the database to something other than your login (e.g., sa). When you do this, you should then be able to add the required permissions to your login.