When I import the path
PolymerLabs/polymer-2-first-element.git I get a generic message that doesn't tell me what went wrong. It's something like "Arg, something went wrong. Try again?". I wasn't sure how to proceed after that.
The solution was to import
PolymerLabs/polymer-2-first-element instead. Maybe Glitch should detect and correct this error, and/or detect full .git links? Other solutions would be validation on the form or mentioning the problem in the error message.
In my case I was copying a GitHub link and ended up with some extra. I'm sure other people have and will have problems figuring exactly what to put in the GitHub import form so it could make their experiences a little better.