Epic Games have released a fix for the issue. Players simply need to restart the client and they'll be able to finish up the challenges!
We’ve released a fix for the following Fortnitemares Challenge: Part 4 - Complete Fortnitemares challenges.
You may need to restart your client to receive credit. Dive in and complete those Challenges!
— Fortnite (@FortniteGame) October 30, 2018
It would appear that the ongoing Fortnitemares event in Fortnite Battle Royale has encountered another issue with the release of the Part 4 challenges.
The fourth and final set of challenges for Fortnite's Halloween event went live on October 30 and looked to be fairly straightforward as most concentrated on dealing damage to Cube Monsters or Cube Fragments.
The final challenge in the list asks players to 'Complete Fortnitemares challenges' and the only way to get past it is to have completed every single task from the four sets of challenges that have been released.
Once you've done that, you'll earn the Dark Engine glider, an item of epic rarity that will be exclusive to players that have completed Fortnitemares 2018 and unavailable for purchase.
Unfortunately, there seems to be a bug affecting the chances of any players getting this final task out of the way quickly, as it doesn't seem to be counting the completed challenges from Parts 1, 2 or 3 at the moment, as shown by 'Flabber-gast' on Reddit.
Unfortunately this is isn't the first glitch to affect the Fortnitemares challenges - a bug in Part 3 allowed players to rack up a staggering 350 Battle Stars for free, and Epic Games allowed players to keep them because it was their own mistake!
"We have just released a fix for some of the Part 3 Fortnitemares challenges that were granting Battle Stars instead of XP. Please be aware, the UI will still show the reward as Battle Stars until the next update. This was our goof and you will keep everything you've already earned," they explained after releasing a fix.
That particular glitch wasn't live for very long, so there's a good chance that this bug that stops you getting your Dark Engine glider will also be fixed nice and quickly!