Sorry to hear you are having trouble with your Twitch Prime loot.
If you have claimed the loot on the Twitch Website and still have not received it in-game after 24 hours, please reach out to our Support team and they can chase this up for you.
From what I can see, you made the purchase for Siege via Steam. If you had tried to purchase the game from our service, it will be blocked due to not meeting the age requirement.
Did you purchase the credits on the same region store as the game you own? If you have not, the credits won't be added to your game due to the regional difference. You would need to contact Sony to enquire about a refund if this is the case.