No, you cannot put a Draw 4 card on top of a Draw 2 card in Uno.
No, you cannot put a Draw 2 card on top of a Draw 4 card in Uno.
No, you cannot put a Draw 4 card on top of a Draw 2 card in Uno.
No, you cannot put a Skip card on top of a Draw 2 card in Uno.
No, you cannot put a Skip card on top of a Draw 4 card in Uno.
No, you cannot stack a Draw 2 card on top of a Draw 4 card in Uno.
No, you cannot stack a Draw 4 card on top of a Draw 2 card in Uno.
No, you cannot stack a Draw 4 card on top of a Draw 2 card in the game of Uno.
No, you cannot put a skip card on top of another skip card in Uno.
In Uno, you cannot put a Wild Card on top of another Wild Card.
No, you cannot put a black card on top of another black card in Uno.
No, in Uno, you must play a card that matches the color or number of the card on top of the discard pile. If you don't have a matching card, you must draw from the draw pile.
No, in Uno, you cannot put a 4 card on top of a 2 card.