During your sync operation, you receive an error message showing specify valid taxcode
on target application, which is SAPB1 here.

Cause

There might be a number of reasons for this error:

• The user is sending a tax code which is not present in the destination application.
• The user is sending a tax code which is different in the destination application (SAPB1).

Resolution

To resolve this issue, follow the links below for reference.

If you are unsure whether the data in source application could be different from what present in SAP B1, we recommend to use Lookups to overcome this situation. Also if you already have lookups implemented, you need to make sure the value coming from source application is added to your lookup repository. 

How to do value mapping in AEC