A system shows this message for the below reasons:
1) If two users try to open/work on the same transaction/room at the same time.
2) Your transaction/room is open and you directly close the browser/Tab.
How to Access: In order to remove the net lock please follow the below steps and video:
Location: Click on ≡ (three lines) >> Net Locks.
Once you click on Net Locks, you will find the below. Just select the room/transaction and click on Unlock.