When this message is displayed on an event hotel on the Front-End, the reason will lie within the room type's inventory page for the block you're searching in.
There are two scenarios in which inventory can cause this message to be displayed.
In the first scenario, the message could be the result of a lack of available inventory. Say we were booking a room for October 14th to October 16th. In the screenshot below, we can see that the day of the 14th has five rooms allocated for the main-block inventory, but all five days have been booked up. Therefore, the room is not available for the specified dates. However, there is inventory for other days, so the room isn't completely sold out yet!
If all room types for an event hotel are unavailable, the event hotel as a whole will show as "Not available for specified dates", as in the image above.
If the guest clicked on "Show Availability" to see what dates were possible, or did their initial search with dates that happened to have inventory, the guest would still be able to book at the hotel. For instance, as in the screenshot above, if the guest moved their dates over one day, October 15th to October 17th, they'll be all set!
In the second scenario, the dates the guest selected may have simply not met the minimum night stay requirement. In the screenshot below, we can see that in this case, the night of the 14th has a minimum night stay of three nights. If the guest changed their check-out from the 16th to the 17th, then they would be all set to book!