Don't be upset, first make sure that you have visited the exact same location as indicated in the task, killed right enemies, with the same condition (if there is any) as in the quest. If you are sure that everything was done perfectly, but this is the task from a paid version of Premium, then do not also forget that the quest must first be received by you, that is, you first need to purchase Premium, only then perform the unlocked task, not vice versa.
If this is also not the case, and you're sure that there's some bug in the game, then please contact our support, while also adding screenshots of the task itself from your Premium, as well as its execution or, which would be even more helpful, a video of how you completed the quest, to your message. We'll be glad to help you!