Modifying entries linked to sales invoices

SOLVED

I'm new to this site.  Our school uses Sage 50.

I'm trying to modify a journal entry (specifically an account #) that's linked to a sales invoice.  But when I click through (from the sales invoice or from the general ledger) to the associated journal entry, one side of the entry is "grayed out", not allowing it to be changed.

Any suggestions or guidance would be greatly appreciated.