You should call phone support, they will need to go through the logs to figure out why. This should be resolved before upgrading.
When I run the precheck on 4 node 4.752 we receive the following error: "Check ESXi version compatibility>. Component ESXi Upgrade Profile of node vxrailhst01.domain.com is incompatible. Current version version: 6.7.0, min_build: 17700523. Expected version version: 6.7.0, min_build: 16316930" Why is is recommending that I downgrade to an older build?
Upgrade from 4.7 to 7.0+ Fails Pre-Check