When a guest says they are attempting to book your listing but see the following message "host's calendar is not updated" it's due to one of two reasons:
The guest has been on the payment page on Airbnb for a while and another booking has been made in that time, hence the dates are unavailable.
Airbnb is not displaying the correct availability (this is very rare). Uplisting checks your Airbnb calendar is up to date every few hours.
Whatever the cause, the good news is this message is appearing as Airbnb are checking the Uplisting calendar for availability and Uplisting is communicating to Airbnb that the dates are blocked. This means it's impossible to get a double booking.