Can't activate iMessages

Try these steps:

Check your device settings

1. 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 number with iMessage and FaceTime. Depending on your carrier, you might be charged for this SMS.
2. Go to Settings > General > Date & Time and make sure that your time zone is set

correctly.

Turn off and restart iMessage and FaceTime

1. Go to Settings > Messages and turn off iMessage.
2. Go to Settings > FaceTime and turn off FaceTime.
3. Restart your device:

• • iPhone
  • iPad
  • iPod touch

4. Turn iMessage and FaceTime back on.

Still not activated after a day? 

It might take up to 24 hours to activate iMessage and FaceTime. If the issue continues after 24 hours, follow these steps:

1. Make sure that your device has the latest version of iOS or iPadOS.
2. If you're using an iPhone, contact your carrier to make sure that you can send and receive SMS messages.
3. If your iPhone is used with a carrier not found in the wireless carrier articles make sure you can send and receive international SMS.
4. Contact Apple Support.

I hope this helps!

Source: https://support.apple.com/en-us/HT201422