Why can’t I edit my text message?

To be able to use the Edit function in the Messages app, you need to send iMessages, and the person who receives the message is also updated to iOS 16. You cannot use Edit for the normal green SMS messages.

You should also be aware that you have 15 minutes to edit before the feature closes.

You can find more information here:

Unsend and edit messages on iPhone - Apple Support

Which iOS version do you have on your iPhone?

Editing and unsending messages on Messages app requires iOS 16 or later.