Getting error while trying to activate iMessage and FaceTime

Contact your Carrier (Cellular Service Provider) The agency who issued the SIM to you to know whether you have a SMS plan enabled at least for One SMS

If your iPhone is used with a carrier not found in the wireless carrier articles, make sure you can send and receive international SMS.

See the pic below

4 Step Process --> Turn off and restart iMessage and FaceTime (The order / sequence is very important)

1. Go to Settings > Messages and turn off iMessage.
2. Go to Settings > FaceTime and turn off FaceTime.
3. Restart your device.
4. Turn iMessage and FaceTime back on (Watch the GIF hereunder)

If you get an error when trying to activate iMessage or ...

Make sure that you're connected to a cellular data or Wi-Fi network. If you're using an iPhone, you need SMS messaging to activate your phone ...