How could I forced WEAP to allocate water to a certain demand site. As I know in LP WEAP decided to alocate water and if it was not successful, we have a shortage. But I want to forced WEAP to allocate water to a certain demand site without including all demand sites.

WEAP can only allocate water to a demand site if there is enough water in the system. For example, if a demand site needs 10 units, but the supply (headflow plus reservoir storage) is only 8 units, then there will be a shortage (it will get at most 8).

Assuming there is enough supply to satisfy this one demand site, you need to make that demand site's priority 1 and the other demand sites have priority 2 or lower. WEAP will then allocate water first to the certain demand site, and then allocate to other demands sites with the remaining supply.