There's an edit session, that you are not connected to, that you want to close.
Find the other session (probably listed to you in the warning when you enter the edit mode):
operator@router> show system users no-resolve
operator p1 172.26.24.43 11:22AM 7 -cli (cli)
Force the other session to log out:
operator@router> request system logout terminal p1
Dealing with uncommitted changes is left as an exercise for the reader.