It doesn’t seem to be invalid, though… See my screenshot above, it even registers use for the key during the rebuild… 
It was generated from my MaxMind account on https://www.maxmind.com/en/accounts/[MyAccountID]/license-key.
Anyway, I’ll try with a new key and see if it works now.
EDIT: Nope, that didn’t work. Generated a new key, added it to app.yml, rebuilt. Still failed in the same way.
I also verified that my account has access to GeoLite 2:
I also accepted their GeoLite 2 EULA again for good measure.
I’m a paid customer of their GeoIP2 Insights service, not sure if that could contribute to the issues?