This requires you to get to Power Level 280. You increase your power level by upgrading the skill tree. Each different point equates to 1 power level and costs 1 skill point to unlock. You get skill points from levelling up throughout the game and get 2 for each level. You can also get some skill points from completing various mysteries throughout the map such as the standing stones which give you 1 skill point for each completed. You can also get 5 skill points for collecting all of Ymir's Tears stones in Asgard and taking them to the offering point.