In order to assign GL Codes to the Main Account, you need to make sure the Cost Allocation Strategy for the account is set to Account.
Go Administration.
Select Accounts.
Click on the spyglass under the Edit column to view the account record details.
While in the Overview tab, find the Cost Allocation Strategy drop-down
Select Account.
Click Save.
Now that the account is set to allocate accordingly, return back to the main Accounts view. You’ll see a green plus icon under the Map column for the account that you just updated.
Click on the green plus icon
Assign the GL codes and click Add.
Click Save.