Since there is a 15 level gap difference, it means that to plunder the level 81, you have to be level 96 or less. Otherwise, you either have wait for him to level when the gap between your level (I guess it is somewhere between 97 to 100) is 15 or less. So if your level is 97, this means you can only plunder people level 82 to 100. When you level to 98, that gap goes to level 83 to 100 and so on.