Why can't I edit a message on iMessage?

To edit a message in iMessage, touch and hold the sent message to give you a new menu. In this menu will be the option to Edit.

See here for more information: How to edit messages on your iPhone - Apple Support

You can only edit a recently sent message up to five times within 15 minutes of sending it.

Was this message sent greater than 15 minutes ago?

Hello~ You have 15 minutes only to edit the message. From the link *void_ranger9* posted…

*You can edit an iMessage up to five times within 15 minutes after you send it.

~Katana-San~