Note that if you only have heroes or keys from Airdrop event, there will restrain for Airdrop them. You, your invitees and their invitees need to have two heroes or keys that get from marketplace or pre-sale to unlock one of your Airdrop hero/key. e.g. If you get 2 heroes (or keys) from airdrop and 2 from pre-sale, finish one mission above once. Then, you are able to mint 4 of your heroes and keys (include 2 heroes or keys get from pre-sale).