Ehm not any? Only smaller areas like MAYBE cities and suburbs aslong as its not a ”huvudled”.... But you wont find any hitchikers there anyway
Why exactly is it not allowed to on ”huvudled”, should not picking up passengers count as stopping and not parking? So no hitchhikers on freeway/motorway but otherwise there should be no problem as long as you can do it without danger to other in the traffic... now, actually finding any hitchhikers in Sweden might be harder.