I also encountered this error. My situation is this: I am testing out provisioning scripts that generate fly.yaml files, launch 4 apps (a dragonflyDB instance and the main app, each for staging/production). For the “main app” apps, I add Sentry using the CLI extension.
I did this a bunch of times, always deleting all the associated apps. After several successes, I became unable to add Sentry to my new apps, and I’ve tried several times. In this state, there are no Sentry projects that are associated with the apps. Seems like there’s some kind of cache that needs to be busted 
?
EDIT: I found a solution: fly ext sentry destroy my-app-name