I have a Galaxy Note 9 and a Galaxy watch and I use Yaata SMS.
You don't get any extra features that you would get in the 3rd party SMS app but you can still read and reply to text messages the same as you can with the default app.
There's a possibility that the short number/code hasn't been registered by the operator.
There are 2 similar cases happen with different operator:
Why won't my phone allow me to send an SMS to a 5 digit number? It keeps telling me error 28 try again. I only activated my SIM card on Saturday.
Not all, if any, short codes are ...