Error Message:
“Account 210000 can only be posted internally in company code BP01″
Analysis:
The settings of GL account 210000 in company code BP01 allows only automatic posting by the system. Manual postings to the gl account is not allowed.
Solutions and Procedures:
If you mistakenly ticked the “automatically post only” check box in the gl account master data or you want to post directly to the gl account, follow this simple procedure:
Transaction Code: FS00
Path: SAP Easy Access Menu → Accounting → Financial Accounting → General Ledger → Master Records → G/L Accounts → Individual Processing → FS00 – Centrally.

Untick the checkbox of “post automatically only” then save the change. Try posting to the gl account manually. Expected result, the error above should not occur anymore.