Welcome to Dominion Online!
That error message usually pops up when you try to play with cards that you do not "own", or with Black Market if for whatever reason you don't "own" enough cards to fill the Black Market deck.
It can also happen if you tried to load a game with cards you don't own, or in some rare other circumstances.
Can you tell me what happened before you got this eror message? Because if you just go into an automatch game, this should never happen.