How can I download iMessage

What happens when you try? Error message(s)?

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

If you can't send or receive messages on your iPhone, iPad, or iPod touch  

Messages Troubleshooting.   

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.      

The messaging app is pre-installed on your device. To enable iMessage, follow the steps in this link: https://support.apple.com/guide/iphone/set-up-messages-iph3d039b67/ios