Here is the instruction from the docs you have referenced in your reply
flyctl postgres connect -a <postgres-app-name>
It is not clear what is <postgres-app-name>. Is it host name or what.
Here is the screenshot of my apps in the dashboard
How do I locate name of the project?
Any attempt to use names listed in the screenshot result in the message content of which is below
Error get app: Could not find App "still-dawn-4253.fly.dev"