I had the same issue for the first couple of airdrop rounds.  The number of characters that you can post in the signature section is limited by your membership level.

At newbie, it is simply not possible to place all the characters that are required for the Junior Member signature.  Please also note that the rules changed to only allow Members and up to apply for the airdrop.

We understand that this may be a disappointment, however, if you continue to meet the requirements, you will rank up to Member in time.