What does it mean when you receive an error that states "Posting prohibited for this GL Account"?

When this error appears, in most cases one of the GL Accounts have the posting prohibited flag turned on.  You will either need to update the GL Accounts in question to ones that are not flagged as Posting Prohibited or adjust the 'Posting Prohibited' flag to NO and update the record if you would like to use the GL Account. 

If you do not want the GL account to be used or anything is posted on it you can mark the GL Account as Posting Prohibited and this will prevent anything getting posted to these accounts by mistake.