The monitor will not join my wifi

The most common reason is the wifi band. PlantSense monitors join 2.4GHz wifi, not 5GHz. Almost every home router broadcasts both, so this usually just works, but a few routers hide the 2.4GHz network or give it a different name. Check your router is offering 2.4GHz and that you picked that network during setup.

Also worth checking, that the password was typed correctly, and that the monitor is within decent range of the router during setup.