This means that session you were in when the crash occurred is still running.
Clearing a Lock on a Purchase Order Following a Receipt Crash
If a Purchase Order becomes locked after a receipt process has crashed, a user with ASC Administrator rights will need to clear the lock by removing any rogue user slots.
Steps
Click System.
Select Maintenance.
Open User Slots.
Click Delete All.
If this does not resolve the issue this will need to go to the support team to investigate further.