Can't delete fields in Custom Object

Have you made sure these fields aren’t referenced anywhere?

Alternatively, there might have been the case where this CO was updated/approved by another user at the same time — resulting in the error. Did you try refreshing and approving it again?

The custom object might be in use by another user or process.

Refresh the page and try deleting the fields again. If the issue persists, check for any active processes using the custom object. If possible, temporarily deactivate them to see if that resolves the issue.