I want a specific card but I can’t request it. Why?

You can only request copies of the cards you already have. In order to request a card from a teammate you need to find it yourself first, in card packs or loot lockers. Plus, you can only request Common or Rare cards. If you need extra copies of Epic or Legendary card, look for them in Shops or Loot lockers.
More Questions:
Why would I give my cards away?
First off, you never lose a card you already have. Even if you have no copies of it, you can still use the card!
Second, there may be cards that you don’t need at the moment, but your teammates do. Remember, sharing is caring!
Third, donating cards is a great way to earn extra coins and level up your New Kid. For each copy of a card you donate, you get Experience and Coins. Each Common card you donate gives you 2 XP and 5 coins, and each Rare card donated gives you 6 XP and 15 coins.