RCS on Google Messages works fine on my Mi MIX 3 using my T-Mobile SIM. Samsung is notorious along with some other manufacturers to limiting features to their stock apps only.
Are you seeing that message through a third party messaging app or the pre-installed one. Advanced Messaging is available through T-Mobile devices, but isn't something guaranteed for devices sold outside T-Mobile channels.
I'm dealing with the same thing on my Galaxy S10+. From what I understand, it's because T-Mobile has their own proprietary Universal Profile which allows for Advanced Messaging. Chatting in Android Messaging won't work even when they release the Universal Profile 1.0 to ALL of its devices because T-Mobile is refusing or just hasn't connected to the Google Jibe Hub like other carriers so that we can all interconnect with one another across carriers. Unfortunately, T-Mobile is struggling with releasing the Universal Profile to their devices even though they've been working on it for a few years. Google knows how slow and slothful carriers are so they are trying to work around this and bypass the carrier themselves. They've begun testing in the UK and France. They announced this earlier in the week and testing began yesterday morning so that's amazing turnaround time. If all goes well there, we should be seeing it soon with no thanks to our wonderful carriers. It's just kind of pathetic and annoying because Google did the work for them with opening up the Jibe Hub to them and was willing to work with them just as they have with those who have connected to it and for whatever reason, T-Mobile just couldn't seem to get on board. How can you not get on board with something like this when you're obviously struggling to make it happen and another company has done all of the leg work for you?! I don't get that. It's frustrating.