iMessage activation keeps saying error

Welcome and thank you for sharing your concerns, aleshinloye164 😉

First, try the following steps:

• Disable any VPN connections and restart your iPhone or tell Siri: "Restart".
• Learn how to update your iPhone to the latest version of iOS.
• Learn what to do if your iPhone won’t update.

Next, learn what to do → If you can’t turn on or sign in to iMessage.

Need more help?

• United States: 1-800-APLCARE.
• Contact Support.
• iPhone Support.
• iPhone User Guide.

Good Luck! 👋🏼😉

Smiliñ 😎 Brian

The complete list for the recommendation to resolve the errors noted can be found in this Support Article:

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

I would also recommend that if you have a VPN installed, remove it. Don't expect that just turning it off will resolve network issues with connections. That recommendation is also seen here where the VPN may be caused by iMessage activations.

If your device has network connectivity issues, check for VPN and other third-party security software - Apple Support

do all 6 steps here, don't miss any out

1.   Settings, Apps, Messages, turn off iMessage  

2.   Settings, Apps, Facetime, Turn off Facetime,  

3.   Power down the phone

4.   Turn on the phone again  

5.   Settings, Apps, Messages, turn on iMessage,  

6.   Settings, Apps, Facetime, turn on Facetime

Then ensure your correct number appears in the Send & Receive