Swipe down from the upper-right corner of your screen (on iPhone X and later) or swipe up from the bottom (on older models).
Press and hold the network settings card in the top-left corner (where you see the Wi-Fi and Bluetooth icons).
Look for the AirDrop icon (a blue radar-like symbol) and tap it to open AirDrop settings.
Select either:
Click the Finder icon in your Dock (the smiling face icon).
In the Finder sidebar, click on "AirDrop" under the "Locations" section.
At the bottom of the AirDrop window, click the text that says "Allow me to be discovered by:"
Select either:
When using "Everyone" mode: