This may be because the recipient's device is not connected to the same WiFi (as AirDrop uses WiFi and Bluetooth) or they are not set to discoverable by you. Additionally, it may be due to the recipient's device not supporting AirDrop.
iPad
- Check that Bluetooth is on
- Check that Airdrop is on as well, swipe up from the bottom of the screen and click on the Airdrop icon so that it is blue.
- Hold down on it and ensure that is says "Discoverable by Everyone"
- Ensure the recipient and you are both connected to the same WiFi. If not take a look at this guide: How do I connect my iPad to the WiFi?
MacBook
- Check that Bluetooth is on
- Hit Cmd+Space and type in Airdrop (or find it in the Applications menu)
- In the bottom of the AirDrop window ensure that is says "Discoverable by Everyone"
- Click on the person you wish to send the file to