First, you need to enter the tax amount in the credit note, and then you can view it in the SST report.
To key in the entry for Credit Note:
Debit side (Sales Account and Sales Tax Account).
Credit (Debtor account).
The taxable amount is under the Sales account.
To show the correct result in the SST reports:
The taxable amount for the CN should be positive.
The CN should be knocked off with the invoice instead of payment.