You would need to use Priority A or B to create a Troll. You only gain access to a particular metatype through a priority where it is listed on the chart.
For example, at Priority E for metatype, the only choice is to be a human.
Now, you do get extra Special Ability points for choosing higher priority, for Troll at A priority, you get 5 to spend. But you cannot play one at all if you don't use at least a B for your metatype Priority