Snap install failure: error: cannot find signatures with metadata for snap

Yeah this is a terribly unhelpful error, I have no idea why they haven't fixed it. It relates to "assertions" in snapd. In this case, that is the verification that the snap comes from a trusted party. Since you're installing a local snap, you have no assertions for it, and snapd doesn't trust it. Think of it a bit like sideloading Android apps, if you've ever done that.

Regardless, you can get past this during your testing with the also-terribly-named --dangerous flag:

$ sudo snap install --dangerous <snap>

note: there was some churn on this flag; in earlier releases it was --force-dangerous.