Chaos Legion Packs in the Shop can only be bought using CREDITS or DEC, there is not a way to buy them directly in SPS.
So to buy packs with SPS you first need to purchase Credits with SPS. To buy packs with DEC press "DEC" and to buy Credits (with SPS) click "Credits" in the shop screen.
Each Chaos pack costs 4000 CREDITS or 4,000 DEC, if your balance is lower than the amount required you will be forwarded to buy the missing CREDITS first. There are 20+ different cryptocurrencies to choose from, so we can select SPS (or DEC) within the available options.
In both cases, you will need the amount in crypto available in your account and approve a transaction with your active key (for example with Hive Signer or Hive Keychain.
Once this transaction is completed, you will have enough CREDITS in your account to buy your Chaos Legion packs in the Shop.
Note: Don't forget to get your Alchemy and Legendary potions to increase your Gold Foil and/or Legendary chances.