@Zsófia5
This is not Airbnb support, but a community of hosts. If you feel it is a bug , then contact Airbnb:
Contact Airbnb - Community Guide [Updated]
I myself extended two reservations this evening and i experienced no errors.
Maybe your settings prohibit the extension you want to do.
Please note the "change" form has two dates (check-in and check-out) mentioned. If you want to extend the reservation, ONLY click on "check-out" and choose new check-out date from calender (must be available offcourse on the calender). Allways keep an eye on the calculated price, as there are known glitches. You can adjust the price manually (click in pricefield), if neccesary.
best regards,
Emiel