@JPR17  Hello, 

If the set up failed, please examine the wifi password first as below:  
Go to Settings-> WiFi on your phone, Tap (or Long press  the router's WiFi name  ->Forget / Delete the network -> Reconnect to router wifi and it will ask for a password, will it accept your router password?

Make sure Kasa App version is up to date 
Check if main router' security type is WPA3-only, if so, please change router's wireless security type to WPA/WPA2-PSK
Use numbers and letters only in router's wireless password (without special characters ) if possible
Try a different phone to set up Kasa device

If still failed, may I know the model of your router and a photo of the Error message,thank you.