Hi @Mark150,
well, these are the new airbnb rules, which are in place since january. You need to either accept or decline within 24 hours - a simple message does not count as anwser !!! - otherwise your answer rate is going down and in certain cases you get penalized with blocked calender and similar, but as far as I know they only block your calendar after you cancel a confirmed reservation. You can read all the regulations in the helcp center.
It also could be possible, that there is another pending booking request which blocks the calendar as well. If this is the case, you need to decline this second request and after this the calendar is free again and you should be able to send out your host offer to the first guest. I had a similar case recently and was able to manage it but needed to first send out the host offer for a different date and then change back to the original date.
Eventually you could send a host offer for another date and then later on change it back to the original date.