The enable secure boot checkbox is invisible, I have met all the prerequisites in the below URL
"You can enable secure boot only if all prerequisites are met. If prerequisites are not met, the check box is not visible in the vSphere Client.
I'm running vCenter 6.5 build 14389939, vSphere 6.5 14320405
Is your host using legacy BIOS or UEFI? Do you have TPM/TXT enabled on the host?
There should be a script on your host that you can run and it will tell you if secure boot can be enabled or not, and may give you more info. Should be located here: