Well...how should one answer the question without posing some question itself?

What kind of mission are you referring ? Do you have a screenshot of your failure notification?

Are you running campaign or a dungeon? What is your team composed of?

As i mentioned, i am trying to 3 star the campaing missions (before beating the last one) and my chapmoins are the basic first 5 i got in game, Ninja, elthan, 2 supports and an archer, same 5 i been running since i started

This isn't an error that I have seen before either, and it doesn't make much sense as you can use any champion in the campaign stages (infact, you can use any champion anywhere).  Can you take a screenshot of the error?  What stage specifically are you trying?

Hmm, in campaign, you can run with 4 champs. If you beat the level, you get 1 star, if you beat the level without losing champs, you get 3 stars. To get 3 stars, you can't run with more than 2 champs.

Otherwise, I have never seen the notification, that you can't use a specific champ in campaign.

I hadn't either... until I tried selecting one of my 5-man saved teams for campaign just now. Then indeed you get a notification saying 'You can't use this Team in this Battle'. 

That would be a good explanation and at the same time a good time to just lol...

You beat me to it! Thanks L9753. :)