How to activate my iMessage

See if anything in here helps.

If you can’t turn on or sign in to iMessage or FaceTime - Apple Support

I’m pretty sure there is an Apple support article for this exact problem. Anyways I have had mine deactivate before if you have used iMessage on your device before just click on a conversation and below their name should be a notification about it click to restore it. If you haven’t used iMessage on your device before signing into iCloud should activate and enable it. If you haven’t logged in that is otherwise contact Apple Support.

crypto97 Said:

"How to activate my iMessage: I can’t use my iMessage"

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Troubleshooting iMessage:

1. Set this Up:

Go Here: Set up Messages on Mac - Apple Support

then...

2. Restart the Mac:

Next, restart the Mac, having just set this up.

Whom to Contact:

Contact your Carrier, as they are the one who provide you with the service of Messaging.