It concerns us to hear that you're not able to send messages to a contact of yours, cmver. Let's gather some details to see what's going on. What make and model phone do you have? Does this only happen with one recipient or multiple numbers?
It looks like I found a solution on my phone. I'm using Android version 10, build QP1A.191005.007.A3. I'm using the default Messages app, version 6.4.044 (Acacia_RC10.phone_dynamic). I'm on Verizon.
I wasn't able to send single, SMS, messages to a certain person. I could send SMS to everyone else I tried, though. I could receive all messages from the person. I could send to the person as a group (MMS) message. I could also send this person pictures, which use MMS. When sending SMS, it would say "sending..." for a moment, and then it would say it failed but I could tap to try again. Each tap to try again also failed.
Here's what I did that seems to have resolved it, and it doesn't involve deleting anything or changing any settings. Open the full contact information for the contact you can't text. Currently, on my phone, I can do it by opening the Phone app (standard app used for contacting people), select Contacts at the bottom, then tap the contact you are having trouble with. This opens the contact's full information. Check that the mobile number listed for the person is correct. It was correct in my case. Then, next to the mobile number for the contact, tap the text in chat bubble icon to open text messaging to that number. After doing this, my 'single' (SMS) texts started going through again to the person. My guess is that some how the Messages app had started using an incorrect or invalid number to send texts to my contact through SMS. My guess is that performing the action I described above fixed the phone number that the Messages app tries to send SMS messages to. In my case, there is another non-mobile number for this contact. This other number also has a semicolon followed by an extension number attached to the end of the phone number. (I'm providing this detail in case it's of any value to any programmer trying to fix this problem.)
I hope others find this information useful.