If you are unable to manually change the status of a reservation, check the source of the reservation. Note you can change the length of stay for a reservation via Guesty only if its source is manual, Vrbo, Misterb&b (Pro users), or if it was made via Guesty Booking Website (Pro users).
In case you are unable to change the status from "Inquiry" to "Confirmed", it may indicate that a confirmed reservation was received for the same dates of the inquiry.