They only way to reach out to hosts on Airbnb is to select their property and dates of travel and inquire of them. What error message are you getting?
Only because I see you've posted another thread about being a property manager, If you are trying to solicit to hosts, Airbnb will catch that, especially if the hosts are logging the communication as spam and stop you from reaching out to hosts. Solicitation is against the Terms and Conditions.