I'm not 100% sure what you're trying to do. I'm assuming you want to a specific instance to destroy itself when if there aren't any obj_pickup1 objects in the game.
First off, instance_count counts all the instances in the game (including the object using the code). Therefore, it's impossible for this code to run and still have instance_count = 0 (since the object must exists to run the code but if the object does exists then instance_count can't be 0).
What you probably want to do is use instance_number. This function checks how many of a specific number of objects are in the room (that are not deactivated).
So in your object that you want to be destroyed, do this:
STEP EVENT:
if instance_number(obj_pickup1) = 0
{
instance_destroy();
}
I believe that's what you're trying to do anyways.