Your Fly org doesn’t have a builder? Strange. What’s the output of fly doctor? If you see anything failing then, try: fly wg websockets enable (ref) and then exec fly deploy --verbose --remote-only ....
You can also force create a (free) builder for your org: remote builder machine stuck in created state - #6 by ray-chen
Something to keep in mind: Such build errors also crop up due to slow / flaky internet connections: Deployments using remote builder failing - #4 by jasmine