ok, so you are not even getting to the point where you are trying to listen. This code tries to connect to the database and only calls listen if that connection succeeded, which it did not.
An unfortunate consequence of JavaScript promises is that the stack tracebacks don’t tell you where to look, but someplace in ‘…/app/models’ you should have a line that reads new Sequelize(...). Can you share that code?
It looks like you are using postgres. If you are using fly’s postgres, that code could be as simple as new Sequalize(process.env.DATABASE_URL). If you are using a database from elsewhere, you might need to set an ssl option, see Using Heroku Postgres From a Fly App for an example.