Apple TV - The wi-fi password you entered is not correct. Try again. (-3900)

Apple TV (pre-4k) ... It will not allow me to change the password for my wi-fi network. I have done numerous factory resets, including a couple via iTunes and my laptop computer.

The only way you can "Forget the Network" is if you can already logon to that network. No other options!

I have tried to enter the Wi-Fi name manually and it still rejects the password that the Apple TV unit has stored in it. The only way I can get this Apple TV to connect to my Wi-Fi is if I hard cable it with a CAT5 cable to the router.

I have only one tool I have not yet tried, but it is tempting. It's called a HAMMER.