L2-DROPBOX CONFIRMATION LETTER -Why?

Hello,

My wife got a L1-B visa based on that I’m trying to apply the L2 visa for me but it is says that i need to use the DROPBOX CONFIMATION LETTER instead of scheduling/appotiment for a fingerprint and others so please let me know why it is happen?

Note: i don’t have any valid/expiried visa (l2 person) and my age is 29.

Thank you,

SR