yarn workspace server start: command not found

That is happening because of this in your fly.toml:

Now a number of mysteries:

  • release is run before start, so in order to have seen start command not found you either had a successful release previously or the release is a recent addition to the fly.toml.
  • It looks like knex is being launched, but is producing no output nor is it exiting.

Tell me more about your database. Are you intending to use postgres? Does fly secrets list include DATABASE_URL? Does your knex configuration match Migrations | Knex.js ?