I don't know that specific mission but - make sure you're doing it on the right difficulty.

Also it might be asking for a certain number of ****** along with the att%, some of those mission take extreme amounts of energy if RNG is agains't you.

Which number artifact challenge is this? 

Kramas is correct, it will require a specific difficulty and rank for whatever attack % chest you are trying to get to drop for you. 

This is a Campaign Mission. The exact wording is this:

"Win a Chestplate with ATK (%) as the primary  stat from Stage 5 of the Sewers of Arnoc on Normal"

And I've done all of that!

Again, just to be 100% clear - the ATK% Chestplate you are winning is from Normal difficulty right? Not a different one?

And also to be 100% clear - you understand the difference between a primary stat and a substat, right?

Threads like this occur very often in the forum. So far we never observed a bug to the missions. The most commen thing is main stat ATK (flat bonus without %) instead of ATK (%). Can you make a screenshot of the item you got?

Was there a requirement for number of stars?

I have the same problem on mine.

Feel free to post a screenshot of you winning said artifact on said level and difficulty.

Yep, would love to see it. Usually just a slight mistake, but it HAS to be Stage 5 in Sewers of Arnoc, has to be Normal, has to be attack % chest that drops. 

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Now it finaly worked after this

Okay so I think the issue here is a misunderstanding on your part. What the mission is calling for is not for an Attack chestplate to drop. It's for a chestplate with Attack % as the primary stat. If you click on the item, you can see what it's primary and substats are. Here's a helpful image, stolen shamelessly from https://plarium.com/forum/en/raid-shadow-legends/673_guides-and-tutorials/105488_basic-guide-artifacts-overview/

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I am having the same problem