For this achievement you will need to have at least 19 skill points available. You then need to head over to the Perks tab and go to the perk that is at the bottom left (if you have progressed through the story a little you may have a bottom row that has gold perks, do not count the gold perks as the bottom row only the purple) you should be hovered over "More Medkits" then you need to acquire the perk, then you will need to hold

and keep purchasing until you have purchased the perk 5 times (this includes the original purchase). Then the achievement should pop.
Video Solution is below