10000488 
Message
Enter G/L account not defined as tax account
Diagnosis
In the Journal Entry window, you have tried to created a journal entry manually:
You have selected the Automatic Tax checkbox.
In one of the table rows, you have specified in the G/L Acct/BP Code column the code of G/L account that is defined as a tax account of a tax group (in
Administration 
Setup Financials
Tax Tax Groups Tax Groups— Setup window Tax Account 
).You have tried to exit the G/L Acct/BP Code column in the Journal Entry window.
Procedure
When the Automatic Tax checkbox is selected, you can only create transaction rows for G/L accounts that are not defined as tax accounts.