The Pope can't declare a crusade because he has no army.
But he could call for one and ask monarchs/heads of governments to go to war.
It's then up to those specific leaders to decide to answer that call or not.
There's very little incentive to go to war for the Pope however.
Most nations are not Roman Catholic so have no obligations to the Pope or Vatican.
Most nations are ruled by elected officials who have to answer to their citizens, rather than being ruled by monarchs.
There is little reward to retaking the Holy Land, and a large portion of it is already in the hands of Israel who are allied with much of the Christian West.
I don't know the workings of the Catholic Church but I imagine if the Pope were to make a sudden announcement tomorrow calling for a crusade then the Church would work quickly to remove him from his position however they could.
Most western nations would also condemn a call for war, especially on religious grounds.