Perhaps you should delete the Account and create new one. Must have been validation application due to Account Type. Create new Account for IGST and set it’s Account Type as “Tax”.
When trying to create a template under sales taxes & charges. when the IGST account is added to the template out of state GST the Rate is showing as 0. Even though in charts of account. It has been defined as 12%
Seems an issue specific to permissions/custmization in your account. Working just fine for me and other users. Can you make a GIF of how are you adding IGST account?