The "PIF is not attached" usually means the network interface selected as the migration network isn't currently active on the target host.
It can happen after upgrades if the host hasn't been fully rebooted.
Worth checking whether a reboot of both hosts changes anything, and running xe pif-list to see whether that specific PIF shows currently-attached: true on the target.
If the PIF looks attached in xe but migration still fails, might be worth a ping to Team-XAPI-Network.