I’m a newbie in Glyphs.
Today I was trying to export my first project (a stencil version of Montserrat Alternates.ttf) and I couldn’t because of this error:
Georg is referring to the current cutting edge release (beta version) of Glyphs 3.2. You can download it by going to Glyphs → Preferences… → Updates, and enabling cutting edge versions.
Am I understanding correctly that you are working based on a ttf file you imported? That is most definitely not recommendable, especially since Montserrat is open source. Work with the Glyphs files available on the repository: GitHub - JulietaUla/Montserrat
After installing the new version, you would need to re-import the .otf as the error is with the import and not with the export. But you should use the .glyphs files from the repo as @SCarewe suggested.