So not to state the obvious here, but have you confirmed that every required item you've equipped is 4* or higher?  If even one of them is a 3 star you're out of the game, regardless of whether its upgraded to lvl 12 or not!

My apologies, but I just carefu;lly reread your initial post.  It sounds to me like that not only does all of the gear you're equipping need to be 4* or higher and level 12 or higher, but each hero must also have at least 3 sets equipped.  So for example, 6 speed items on one hero equals three sets, or one offense, one speed one crit rate etc.

Hope that helps!

Posting screenshots of those 4 champions will help.

As you worded it, you could use 4-set pieces like 6 Lifesteal artifacts, that doesn't work.

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Thank you for your responses. here are photos of one of the champions. the artifacts are all from the same set "Relentless". i also have 3 other champions with similar criteria, all the artifacts are from the same Set and all are at level 12 or higher. your help is appreciated. i look forward to your comments, thank you...

you are not supposed to use relentless, try the defense, offense, resistance, accuracy, speed, life, critical rate or critical damage sets only

You are overlooking that they need to have 3 sets. That doesn't mean 6 pieces, it means what it says: 3 sets. Obviously, with only 6 spaces, if you need 3 sets it means the sets can only be 2-piece sets, such as the ones Kalindra mentioned. 

On a side note, are you aware that having 6 pieces of a 4-piece set like Relentless is kind of pointless? The 2 extra pieces won't give any kind of bonus. 

Thank you for the infomation, i now understand the concept of "3 Sets". 

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Switched out what i thought would be sets but still no indication in the Challenge Artifact 8.7. can you tell me what i am missing. thank you...

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artifacts from the above sets only.

What you put now is each time 2 pieces from a 4 piece set - so you don't get a single set bonus. You need 3 sets, so also 3 bonuses. The ones Kalindra shared, or other 2 piece sets like cruel, immortal, divine speed/life/... and so on. 

Thanks again and for your patience, i was able to finnally get one to register. i have only been doing this for a couple of months but mostly on my own and obviously not understanding some of the subtle points of the game....

No problem. Those progress missions or challenges don't normally bug, but they can be misleading if you don't read and do precisely what it says. 3 sets means 3 sets, not 2 sets, not 3 parts of sets. 

thanks again for your help

I was having trouble with this challenge as well but I just completed it thanks to Kalindra. You need 4 champions equiped with 4* or higher gear at level 12 from either Life, Offensive, Defense, Speed, Critical Rate, Accuracy, or Resistence. Make sure a champion has 4 stars or higher and is equiped with the entire set of the ones listed here and you will complete this challenge.