My iMessages has been saying "Waiting for activation..."

Device or computer?

IOS

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

Some things you can try. Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch         

Try a forced restart.

Reset Device.      

If necessary

Hard reset.        

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      

Restore - If you see an error when you update or restore your iPhone, iPad, or iPod

Backup - Restore your device from an iCloud or iTunes backup.      

OS

Message activation error.    

Error Occurred During Activation for iMessage or FaceTime on Mac

Someone else may chime in here with a more definitive answer, but here are my comments.

I saw this problem when I migrated my wife into her new iPhone 12. In my attempts to hasten activation, I deleted her phone number and then had a hard time putting it back. After quite a few forced reboots, and with my wife angrily watching over my shoulder, the phone number magically activated. I would recommend that you reboot the phone once or twice and then leave it in "Activating..." status overnight. Good luck.