This is caused either by an existing user and will clear once they have finished or by a session that has failed.
Examples include:
Table KIT in use by another user - aborting not allowed - will retry on pressing OK
I receipt stock and getting Message 2637 the order is being received by another user.
I amend a Purchase Order and getting Message Order is being received by another user, try later
I try to book some material into stock and can't because it says that the order is locked by another user.
Make sure no user is actually trying to process the record with the in use message
Ask your system admin to clear the user slots in Supply Chain:
Select System tab
Click on Maintenance
Select User Slots
Click on Delete All (It will only remove inactive sessions)
Select Yes when asked OK to delete?