If you want to play Viceroy by yourself, you can play against a virtual opponent who acts only during the auction phase.
Set the game up according to the usual rules (place 4 gemstones of each color in the reserve and take your initial gemstones and cards).
Each turn consists of the auction and development phases as usual. After you make your bid during the auction, take one of the unused gemstones out of the box without looking at it. This will be your virtual opponent's bid.
If it is the same as yours, place your bid into the reserve and your opponent's in the box and move on to a new auction following the same rules.
If your virtual opponent chose a different card, you receive a card that matches your bid as usual, and the card your opponent is supposed to receive is discarded (if your opponent can choose one of two cards, they always discard the card at the base of the arrow).
If your virtual opponent chooses a color that does not match any of the character cards, discard a card of your choosing from those left at auction after you receive your card. Always return all your virtual opponent's bids to the box and make sure these gemstones do not end up in the reserve.
If you play by yourself, you cannot lose. Calculate your points after every game and try to beat your high score.