In spite of subscription to Visio Online plan 1, Field Mapping options does not allow edit of diagram.
Edit of Visio Visuals after subscription to Vision Online plan doesn't work. Whats going wrong here
Shakun Grover commented
Editing should ideally work if you have Visio Online Plan 1 license. Did you try after a few minutes of getting the license? To help you further on this, please connect with us at tellvisio[at]microsoft[dot]com.