How do I send an SMS to a contact that only has an e-mail address?
I just tested the issue with a few different messaging apps, and I came to the conclusion that the app was simply not made to do that. In both Google messages and textra, I had the issue you describe, but as soon as I tried it in Samsung's messaging app it worked perfectly. As soon as I started typing the name, it gave me the email address as an option. 

That's why I conclude that Google messages was strangely not made to do what you are looking for.

As a workaround, I suggest that you text the person once, and then every time you want to text them again, instead of initiating a new message, simply press on the search icon on the top right of the screen and type in the name of the contact, press on the contact, and you have achieved the same thing as you wanted with ease.