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.

When all allocated inventory for a block has been booked up, across all possible dates, the room type will show up as "Sold out." When all room types are sold out, the event hotel as a whole will show as "Sold out", as in the image above.

Allocating more inventory to the block will have the event hotel show up as available once again, but if you do so, make sure you aren't allocating rooms beyond what's been decided in the hotel contract!