In my experience it can also happen if you copy a deck code from somewhere (or change a wild deck to standard). I found the best fix was to remove all the cards and manually re-add them. In particular I have found e.t.c to be a prolematic card so I would start with him if you've got it in your deck
every time this pops out
That used to happen if you have card(s) in your deck that are banned in the format you are trying to play in.
If you see a bad post on the forum use the report function under it, so I or someone else of the moderation team can take care of it!
In my experience it can also happen if you copy a deck code from somewhere (or change a wild deck to standard). I found the best fix was to remove all the cards and manually re-add them. In particular I have found e.t.c to be a prolematic card so I would start with him if you've got it in your deck
nothing mentioned here , this is my kill me deck that i complete weekly quick its wild card self dmg nothing banned , dont have this problem before
https://hearthstone.blizzard.com/deckbuilder?deckcode=AAEBAf0GAr2+A72+Aw6/FJPkA+SKA+O+A/ukA4jvA531BsSeBs4Hw7gG8tAChKAE184DzuQFAAA=
Elementium Geode & Hecklefang Hyena are good inclusions
DJ