Unable to equip the Drakefire Pistols

You need to simply select a difficulty and the button will become enabled as per this source:

You have to click and actually select a specific difficulty

The 100 hero power cap in the difficulty menu does mean that the boxes you get as rewards for clearing missions will only drop items with at most 100 power. This only includes the boxes you get directly from the mission, not any other boxes such as from leveling up a hero. Also, as box rewards are usually near your best items in power, you will not immediately get items even close to 100 power on a fresh account.

Separate from the cap the player is told about, the lower difficulties have a soft and hard cap on power that are not told to the player. For Recruit difficulty the soft cap starts from 200 power, and there's a hard cap at 600 power. Having 600 power on Recruit will only act like actually having 300 power. https://vermintide2.gamepedia.com/Hero_Power