Blog Template: can't create new templates

I keep getting the error message "Whoops! The blog ID you posted is incorrect. You cannot template the main site. Please go back and try again. (Generated by New Blog Templates)"

I have tried to template a page on the main site (https://legacyness.com/journalist-home-default/) as well as a subdomain (https://template.legacyness.com/home/) and got the same message both times. Where exactly do I have to put it to save a template? Thanks