If you can't  see the device that you are trying to AirDrop, check the following:

• Make sure both of the device have WiFi and Bluetooth turned On. If you have your Personal Hotspot on, turn it Off. 
• Make sure the device you are sending is nearby. You can't AirDrop to the users who are out of Bluetooth and WiFi range. 
• If the other person's AirDrop receiving setting is set to Contact Only, and you are not in their Contacts, set their Airdrop Receiving Setting to Everyone.