Can't generate font atlas

Hi, community!

I have a quick newbie question about TextMeshPro

I've downloaded 100% free for use font from one of the sites that were noted in an educational course which I'm going through now (https://dl.dafont.com/dl/?f=punkassbitch) and wanted to create font asset but unfortunately, there are 2 problems:

1. TMP importer is empty (even though it was highlighted in the course to launch import, but maybe this is OK as TextMeshPro was right from the beginning when I've created my project)
2. (Which is more important) after moving .ttf file to "Fount Source" section - "Generate Font Atlas" button is inactive


Please support me with how to fix this issue.

Thanks in advance.