I believe it's the "Win a boots with ATK% as the primary stat from Stage 6 of Sewers of Arnoc on Normal" progress mission.
I started a new account about a week ago and currently a few missions away from getting the 300 gems, so had no issue with any of these, though it did take awhile to get one to drop.
Are you certain that you are on normal difficulty on the campaign (the lowest difficulty) and not hard? The boots that drop from Stage 6 need to have ATK% as their primary stat, which can be frustrating if the game doesn't roll it. The boots also needs to actually come from this difficulty and stage, and not anywhere else (e.g. Marketplace)
Yea, as Matrim said, the boots need to actually have attack % as their primary stat. When a boots is given as a reward, it can essentially have one of 7 odd primary stats, so if it spawns with another stat type then it won't count. Depending on luck, you could get it immediately, or need a lot of runs to get it