There seems to be an error in the setup that needs to be done in order to calculate the Deemed Value for GST Base Amount in Foreign Exchange Transaction.

It Requires G/L Account to be selected in Account Type field for Customer or Vendor Transactions on the Bank Payment Voucher. This requires to have direct posting on G/L Accounts in case of Customer & Vendor which defeats the overall purpose.

It should calculate even when the Account Type is Customer or Vendor. This way the transaction is recorded by following the normal Posting Setups.

https://learn.microsoft.com/en-us/dynamics365/business-central/localfunctionality/india/gst-bank-charges-transaction