I tried repeating VMware KB: Configuring Network Time Protocol (NTP) on ESX/ESXi hosts using the vSphere Client in ESXi 6.5 using the web-client (the steps are very similar, see [WayBack] How to configure ESXi 6.5 Network Time Protocol (NTP) via Host Client? | ESX Virtualization).
It failed with the non-descriptive “Failed – An error occurred during host configuration.”:
Viewing the details isn’t of much help as you do not get extra information: