credit memo in Receivables and applied GST with 18% on this credit memo. But The amount is not effec

Created credit memo in Receivables and applied GST with 18% on this credit memo. But The amount is not effecting on this AR credit memo. We can able to apply the credit memo on invoice using the same tax code whereas for credit memo it is not able to apply. Can any one clarify that on credit memo GST amount will not be effected automatically?

Thanks,

Jagadeesh Mandava