Hello
It looks like the ESXi host has been added to vCenter Server with the name "esxi" which is unsolvable by Converter machine. I'd recommend, if possible, to re-register the host either by a dns name or by IP. If that proves a problem, add 'esxi' in converter's machine hosts files for a workaround.
HTH,
Plamen
P.S.
If you go the way of the workaround and have also Windows machines to convert, apply the hosts patch to the source Windows machines, too.