Universal Clipboard not working between iPhone and Mac?

Use plain text you can recognize. First paste it on the same device to check that the copy worked. Then copy it again and paste into Notes on the other device without copying anything else. The shared clipboard only holds the item briefly, and a new copy on either device replaces it. Repeat in the other direction if you normally paste from Mac to iPhone.

On iPhone, open Settings → General → AirPlay & Continuity and turn on Handoff. The menu may be called AirPlay & Handoff. On a recent Mac, open System Settings → General → AirDrop & Continuity or AirDrop & Handoff, then allow Handoff. On older macOS, look under System Preferences → General. Check the Apple Account and both radios on each device.

Retry the original content immediately after copying it. If it fails only in one app, try another destination that can accept that type of content. For several files, use a file transfer method such as AirDrop. If even plain text fails in both directions, check Apple’s compatibility guidance, restart both devices and check for updates. If a VPN is active, check whether its configuration blocks local networking.