Tax number 3, Tax number 4 in one-time vendor document

Hello Expert,

At first I post one-time vendor document through T.code FB01. Later I want to edit tax number 3 (stcd3) & tax number 4 (stcd4) through T.code FB02 but they are greyed and not allow to change them. I’m already check T.Code OBD3, OB23, OB24 but nothing’s change.

Please tell me what can I do to fix this?

Best Regards,

Jiranan