I have a similar issue, but doesn't seem like the same issue?
On my Mac (macOS Sonoma 14.7) trying to send a message to 1 android user it just fails. His messages get to me no problem, but I have to reply on my phone. I have so far not had ANY issues texting other android users since updating my phone to 18.0.1 yesterday. on my phone, when I send him messages, it says "Text Message • RCS" and when I send the message from my phone it also fails every time. I then need to tap the exclamation point and select to send it as SMS in order for it to go through.
1 other android user I messaged today still say "Text Message • SMS" but another one I messaged also says "Text Message • RCS" and that one went through fine?
Does this just mean that the person I'm messaging on android doesn't have an RCS compatible phone? I assume it isn't his carrier because if his carrier didn't have it, wouldn't is still say "Text Message • SMS" like for my other android friend?
All that being said, it would be nice if the messages app on my Mac would at least give me the option to send as text message when it fails!