I clicked on New Tax Code > Got a list of inbuild Tax Codes.
Found INDIAN GST New Rates. Selected 1 of them and clicked Create.
Then on the newly created Tax Code, clicked Edit.
I found something wrong was opening up. I could see some other ledgers appear in the Tax Code Combo.
Not sure if that’s a BUG or something is wrong in my installation.
Except if you had already created a custom tax payable account, you can not edit in-built tax codes. Please read the guide: Create and use tax codes | Manager