The Samsung watch can only use the stock Samsung Messaging app on the phone to have full integration like view past messages and send messages via the actual “Messages” app on the watch.
Note, The Samsung messages app is NOT compatible with Republic, so you can not use it if you are with RW.
You can not use other 3rd party messaging apps on the Samsung Tizen watches and have it work natively with the watch as one may expect the Samsung Messages app does.
This is a well known limitation and design of Samsung watches.
They really want you to use it with a Samsung phone and Samsung apps only.
You can, however, respond to notifications from those 3rd party apps via the watch like reply to the message via text, voice, or auto generated messages, but you can not open the Messages app on the watch and view past messages.
I use Pulse SMS, and previously used Textra, and the notifications reply work fine on my Galaxy Watch, with no odd error message.