Hello @Jenny1273
oh dear what a mess
If the guest wanted to extend the current booking during a live booking I didn’t think you could do an amendment but you or the guest would need to contact Airbnb to get them to do this for you.
Sadly as you didn’t block the date and you have your settings for IB there was always a risk that someone would come along and book the dates.
Even more unfortunately you then cancelled the new guests booking meaning you are liable for the cancellation fee and have your dates blocked - did you not see the warnings Airbnb gives you about this when you went to cancel the booking?
You can either have the guest pay you directly for the extra day or call Airbnb and see if they will be lenient and unblock the date.
You may want to remove your IB setting while you sort this out.