"Journal has already been posted and, consequently, is not open"

Cause:

The cause of this error is when you are trying to post a Journal which is already posted.This mostly happens when you are trying to post using X++ code.

Check if you are passing new record(LedgerJournalTable) to Class LedgerJournalCheckPost everytime you call it for posting.

First time you call the class the selected record gets posted but second time it is stopped in the validation process.As the record is already posted once.