Cannot rent a plane -- "It's in air currently!"

Hello everyone,
I rented a plane this morning (a Beechcraft C350i, see Log in - Aviatife.com - run your own airline company!), but I could not start a flight yet; if I try to start a flight, the system tells me that

“You cannot fly this aircraft. It’s in air currently!”

I tried both this morning and a few moments ago and the error was the same.
What could I check?

Regards,
Lazzarocammina