When trying to live migrate VM's and you encounter this error, it means that the hostname of the ESXI host could not be resolved properly.
This can be because the DNS server is unreachable/offline or you recently changed DNS servers and the ESXI host doesn't know it yet.

If you recently changed DNS servers and you are 100% sure it's online and working, try restarting the VPXA service on the EXSI host first.
You can do this by logging in the ESXI host by SSH and entering the following command: /etc/init.d/vpxa restart Then try to live migrate again.
Some hosts have SSH disabled by default, you can enable this by logging in the host directly and click Manage > Services and check if TSM-SSH is started or not

If that doesn't work, then DNS resolution seems to be really broken. You can fix it or go for this quick fix

1. Login to the ESXI host by SSH
2. Navigate to the VPXA config folder using the following command: cd /etc/vmware/vpxa
3. Make a copy of the current config file just to make sure with the followign command: cp vpxa.cfg vpxa.bak
4. Now you can edit the VPXA config file by using the following command: vi vpxa.cfg
5. Scroll down to the bottom of the file and look for the line <hostIP>HOSTNAME</hostIP> replace the hostname with the IP address of the ESXI host
6. Save the file by pressing ESC and typing in :wq!
7. Restart the VPXA service so the new config gets applied with the following command: /etc/init.d/vpxa restart