The mobile device is not connected to the same network as the one the INPLUG is trying to connect to.
If you have two networks ensure your mobile device is connected to the network nearest to the INPLUG. It is better to turn off other networks if possible to stop any conflicts during the setup process.
You have entered an incorrect password.
This is a common issue; make sure the password is correct, tick the box that allows you to see the password before you proceed further.
You are trying to connect to a 5G network.
The INPLUG will not connect to a 5G network only a 2.4G connection; this is the case with virtually all all home automation devices. 2.4G networks have a longer range than 5G.
All routers that have a 5G network will also have a 2.4G network. Make sure you give them a different SSID (network name) so you are clear which network is which.
The mobile device you are using for setup must also be connected to the 2.4G network during the setup process.
Check whether the Wi-Fi indicator light on the INPLUG starts flashing slowly after you press "connect" on your mobile device.
If it does, the configuration process has been activated and the device is trying to connect to the router
If the WiFi indicator light keeps flashing rapidly and does not enter the state of slow flashing, then the configuration process is not activated. Check that your mobile device is properly connected to the router via the correct WiFi network.
Sometimes resetting your router will solve this issue.