The tax added feature is not available

The tax added feature is NOT AVAILABLE for SUBSCRIPTION and DONATION based payment types.

Is it also not available for USER DEFINED AMOUNT? Is there a way I can add tax to USER DEFINED AMOUNT?

Thank you for your assistance.

You can achieve this using regular field + form calculation + payment field.

You can use the Get Price From option of User Defined Payment type to get actual calculated amount from the Calculation field.

You can test this on my demo form https://www.jotform.com/83211686508964

Let us know if you need further assistance.

Thank you!