Check the security profile associated with the user, they will not have the ability to post Nominal Journals. If they should be able to do this, edit the profile and switch on.
Click System, Profiles
Select security profile user affected
Click Edit
Expand Nominal Ledger
Expand Transactions
Tick Journals
Click Save
User needs to log off and back into Financials for changes to take effect.
If the Journal was created by a user who has since been moved to a LEAVERS profile, or one without permission to post Nominal Journals, you will need to create a new Nominal Journal and copy and paste the data from the old Journal into the new one.