The error "The journal does not balance. Have you mapped all relevant nominal codes?" means that you have one or some nominal codes that are not mapped to your software chart of accounts.
How to fix it?
Scroll down to the "journal entries" section from your pay run details page and look for lines with warnings. They are notified by the icon danger and the text "code missing".
Go to Profile > Settings> Accounting
Click on "Map nominal codes" under your connected accounting software logo.
Scroll down to the section called "Optional codes".
Find the missing pay code
use the dropdown menu to open the list of accounts from your accounting software
Select the account
Scroll down and click on "update."