Enable AirDrop:

Control Center > AirDrop > Everyone or Contacts Only. Verify enabled on both devices.

WiFi and Bluetooth:

AirDrop requires both enabled. Turn on WiFi and Bluetooth on both devices. Don't need WiFi connection, just enabled.

Disable Personal Hotspot:

Personal Hotspot interferes with AirDrop. Disable temporarily to test.

Bring Devices Closer:

AirDrop has limited range. Maximum about 9 meters. Move devices closer together.

Toggle Airplane Mode:

Turn Airplane Mode on then off. Resets WiFi and Bluetooth. Then try AirDrop again.

Sign into iCloud:

Ensure signed into iCloud on both devices. AirDrop to Contacts Only requires iCloud.

Conclusion:

Setting AirDrop to Everyone bypasses most issues. WiFi and Bluetooth must be on. Personal Hotspot is a common problem.