Hi @Nathalie2536 , did you manage to connect with the host? As @Shelley159 mentions, sharing a screenshot would be helpful in this case to better understand the error.
In this article about 👉Contacting hosts before a reservation 👈 , it says :
Before a reservation
You can check out the listing and click or tap "Contact Host" to send a message. Pro tip: If you want to ask about specific reservation details, enter the dates and number of guests for your trip before hitting "Contact Host" (you won’t be able to send a message unless you have dates selected).
Are you pressing the "contact button" without selecting the dates?