gcloud beta functions command says "Invalid choice functions"

The functions are in subscription-only beta phase right now. From Quickstart:

Beta

This is a Beta release of Google Cloud Functions. This API might be changed in backward-incompatible ways and is not subject to any SLA or deprecation policy.

Want to get advance access to the latest and greatest? Click here to become a beta tester.

The gcloud cmd might be checking beta subscriptions. Did you subscribe?

Also make sure you have the most recent cloud SDK version.

Make sure you have the latest cloud SDK installed.

I got the same error and it seems I had version 137 while the latest is 152.

After running gcloud components update it was solved.