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From this error message, it looks like your tenant uses New Universal Login, but has not set up the custom domain.
For New Universal Login to be able to set templates, it needs a Custom Domain to be set up and can be customized only via our Management API. This document explained the details.
You are right, I was missing the custom domain. And one more question, If I want to reset the template(Disacard any changes I’ve made), just use Delete api method? or another way will do it?