If the category Code is either E or Z 0 then category code reason cannot be null

Cause: 

This error occurs, when the tax type of the transaction is something other than Taxable tax type.

Reason:

You have to fill in the category code reason with valid details.

1. Open the stock item or ledger in the alteration mode
2. Under Statutory Details, select the Reason Code and Reason Text for Reason for tax exemption/zero rate.
3. Press Ctrl+A to save.