This normally is an error that occurs if a user is in the postings screen, follow the below to make sure that everyone is out of the accounts/postings.
Check that you have the posting screen closed not just that a posting is not been done the screen can not be open.
Check to make sure that no other user has this screen open or doing any postings at the same time.
If everyone is out and it still doesn't work it might be a user is stuck in a module and this needs resetting via System Use.