After the latest upgrade it’s not possible to play in Pandora mode anymore if you don’t have all DLCs.
It allows you to form the deck and then drops “You cannot play with this deck because it contains card XXX from a DLC you don’t own”.
If that’s not a bug, and I think it’s not, it could at least to note about missing DLC during deck selection not to make it so annoying (or that was the aim?).