vCenter Server 6.0 cannot manage ESXI 7.0
vCenter Server 7.0 can manage ESXi version up to 6.5 hosts in the same cluster as ESXi 7.0 hosts.
vCenter Server 7.0 cannot manage ESXi 6.5 or earlier hosts.
vSphere supports upgrades from vCenter Server 6.5 and later to vCenter Server 7.0. To upgrade from vCenter Server 5.0, 5.1, 5.5, or 6.0 you must first upgrade the vCenter Server instance to version 6.5 or later releases, and then upgrade to vCenter Server 7.0.
in your case it is a compatibility issue! You can use ESXI as a standalone host!