Hello, I have a question about weakness.
For example, Forcible Engery feat :
... you can select one target that was damaged to gain weakness 5 to that damage type until the end of your next turn...
This has no effect on creatures with resistance or immunity to the energy type you choose.
Does a target with a resistance or immunity gain the weakness ?
In general, it is very possible for a creature to have both a weakness and a resistance to a type of damage. Usually they won't have both natively - one will be being applied by some other effect.
There is an order that they are applied in for when a creature has more than one.
It doesn't actually say so, but I think that if a creature has immunity that weakness damage won't be added afterwards - because weakness only triggers if the creature actually takes damage of that type.
Forcible Energy has a line of text saying that if a creature already has resistance or immunity to that type of damage, that the weakness does not get applied. But if you cast Barkskin on a Charhide Goblin, then they will end up with both fire resistance and fire weakness.