Apollo on android: can't download schema or generate classes

I'm trying Apollo and graphql for the first time and I want to use this api and I'm following the official example here. However

running the schema download task

gradlew :app:downloadApolloSchema --endpoint='https://countries.trevorblades.com/' --schema='app/src/main/graphql/com/trevorblades/countries/schema.json'

results in

 Execution failed for task ':app:downloadApolloSchema'.
> Expected URL scheme 'http' or 'https' but no colon was found

with both the repo I want to use and the example's.

If I try to manually download the schema using the button on the repo's page, place it in the correct directory and run

gradlew generateApolloSources

it results in

 Failed to parse GraphQL schema introspection query from `[...]app\src\main\graphql\com\trevorblades\countries\schema.json`

again with both my repo and the one from the example.

I've the feeling that I'm missing something really trivial here, but I really can't figure it out.

any help will be appreciated.

from Apollo on android: can't download schema or generate classes