What does "can't deliver, user unavailable" mean on Messenger app? Currently facing a complicated issue

Aight, so this is a bit complicated as mentioned above

Basically, I tried to DM a person on messenger, but I got a "Can't deliver, user unavailable" beside my texts with a yellow exclamation mark.

This left me very confused for a bit. I thought I got blocked, but when I searched the person's profile, I could still see it. I also attempted to block that account, and I even succeeded in doing so. I was also able to see the "Active now" on their profile when i manually searched the profile on FB and clicked on the messenger icon there and i was even able to see that my previous messages that i was able to send successfully quite sometime ago was seen but it was the complete opposite on the app, there was no active status and my previous texts before this issue was "sent". Im not a very tech-savvy person, and I don't know how any of this works. If I really got blocked, I would not be even able to search their profile and find it, let alone, manually blocking it. So, is it possible to block a user who has blocked you on FB?

I'm so confused. Please help. What does this really mean?

UPDATE: The problem seems to have been solved in a very weird & random way. I'm able to receive texts from the same person and message them back ONLY if I log into my messenger account on another device, although I'm not able to send photos and videos, unfortunately. The person's chat doesn't even show up in main page of messenger in my own device despite having a conversation with them previously on another device after the incident, nor can I message them from here, but oh well, it is what it is.