To resolve this issue, VMware Update Manager should be upgraded. The minimum version required should match the version of the ESXi image you are trying to import.
For example, ESXi 6.7 upgrade image require VMware Update Manager 6.7 to install. This may also require vCenter Server and other components to be upgraded, Same applied to later versions.
Note: With Update Manager 6.5 you can upgrade hosts that are running ESXi 5.5 or ESXi 6.0 to ESXi 6.5. Host upgrades to ESXi 5.0, ESXi 5.1, ESXi 5.5, or ESXi 6.0 are not supported. ESXi images with these versions will not be accepted by Update Manager. Similar, Update Manager 6.7 will only accept ESXi 6.7 installer images and Update Manager 7.0 only ESXi 7.0 installer images