Hidden SSIDs "<length: ?>"
Many aireplay-ng commands require knowing the SSID. You will sometimes see <length: ?> as the SSID on the airodump-ng display. This means the SSID is hidden. The ? is normally the length of the SSID. For example, if the SSID was test123 then it would show up as <length: 7> where 7 is the number of characters. When the length is 0 or 1, it means the AP does not reveal the actual length and the real length could be any value.
To obtain the hidden SSID there are a few options:
Wait for a wireless client to associate with the AP. When this happens, airodump-ng will capture and display the SSID.
Deauthenticate an existing wireless client to force it to associate again. The point above will apply.
Use a tool like mdk3 to bruteforce the SSID