I am getting the following error We can't reclassify the tax code as you selected a tax code without a purchase rate for a purchase transaction.

Thanks for posting in the Community, Azeem. 

There was a reported incident with the inability to reclassify GST on deposit transactions and getting an error "We can't reclassify the tax code as you selected a tax code without a purchase rate for a purchase transaction." As per the last update, this is not a bug and the system works as designed. However, we have raised an enhancement request for a better experience.
 
Furthermore, if the transaction is of purchase type and you are trying to change it to a sales type GST then it's not allowed. Also, if the transaction is of sales type and the user is trying to change it to a purchase type GST then it's not allowed.

As a workaround, you'll have to change the tax code per transaction manually.

Drop by again in the Community if you have other questions. I'm around to help you.