I have Google messages installed on my watch because the Samsung texting app is terrible.
If I ask Bixby to send a text message, it tries to send from the Samsung texting app on the watch for even though Google messages is set to default on my watch and my phone.
I get a message saying "text could not be sent."
Samsung, I am begging you get rid of Bixby and get rid of your terrible texting app. Google already has these covered, I don't why you think they're even necessary. It's a waste of resources and our time.
Just use Google Assistant.