Better yet just ask Google it would've took you about the same amount of time as making this post
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Yes, you can use Steam credit to buy a Steam Deck:
Steam gift cards
You can use Steam gift cards to purchase the Steam Deck through Valve's Steam client. Steam gift cards are available in digital and physical formats. You can buy physical gift cards at retail stores, or purchase digital gift cards from Steam.
Steam Wallet codes
You can use Steam Wallet codes to add credit to your Steam Wallet, which you can then use to purchase the Steam Deck. You can buy Steam Wallet codes at local retailers, or receive them via SMS, email, or on receipts.
You can purchase the Steam Deck directly from Steam in the US, Canada, UK, and EU. Steam is also working with Komodo to sell the Steam Deck in Japan, South Korea, Taiwan, and Hong Kong.