Hi
Let me start by mentioning that the suggestions in nachogonzalez' answer are misleasding (sorry). Converter 6.2 works with RHEL 5.1 (VMware vCenter Converter Standalone 6.2.0.1 Release Notes). It is not supported because Converter is end of support, but it works. The KB article linked is about RHEL/CentOS 7.6; it is related to an issue that RedHat seemingly have fixed later (i.e. 7.7 is expected to be convert-able again)
The error in the message indicates that GRUB has failed to install. Converter calls the source VM's grub-install comand (after cloning all files). Something has failed in that call. One thing you can check is whether such executable exists on the source and how it behaves, Be careful since it can be a dangerous command to execute! Executing with '--version' or '--help' option is innocuous.
More details could be understood from the helper log. You don't really need to enable logging in helper. Just export the log bundle from Converter UI (right clink on the failed task) and dig in to find the helper log. Drill through worker and agent tasks to find it and attach it here for examination. Alternatively you may attach the whole bundle.
HTH,
Plamen
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