It would be possible in old gwent, when every turn was literally just playing one card, but I think now it's too hard to implement properly.
There is a problem with Orders that can be activated in any order (he-he), and moreover, cards can be played in any time between them. If you would have an option to undo your last action, your opponent would always see one action less then it is until the end of turn, that also could be ended by rope rather than by player. I'm sure that such an option will cause many weird bugs in any situation that is more complex than just playing a card and pass.
Also it will spoil the games pacing, you always will wait for your opponent more then needed.
But I agree that there's a big problem with missplaying. Row highlighting just don't work, I just can't recognize it when I play a card. Targeting cause problems some times, especially if you play with gamepad, but I think it can be solved in other way.
I think it's a good idea to highlight "right" row in a contrasting color (orange would look great) and for cards with two abilities (such as Yen) highlight them with two different colors (for example, red for "atacking" abilities and green for "defending". though I feel like these two colors not very good for gwent's color scheme).
Problem with wrong targets could be solved by adding "apply" button instead of "undo". To perform an action you need to place a card, choose target if needed and then hit Apply (if card has row-locked ability and you place it in a wrong row, you still need to apply your action). If turn ends before you apply your action, it applies automatically. Also, it should be optional feature that you can turn off in settings menu if you don't need it.
This way your opponent don't see your actions too, but now he can't see them because they're not happened yet and not because server hides them.