Hi himal,
what is your version of Windows OS?
You need to Save and Run the Installer or Install.
what is your version of Vsphere client?
The Windows Client isn't (has never been) supported for ESXi 6.5, although the v6.0 cliient may have worked.
In order to manage a v6.5 host, please use the Web Client https://<esxi>/ui/
André
As a.p. said, this has never been supported. The fact that it worked was basically a "bonus" until now. To manage ESXi 6.5, you need to use the HTML5-based host client accessible as a.p. said above.
Just found this in the VMware ESXi 6.5 Update 1 Release Notes:
vCenter Server, vSphere Web Client, and vSphere Client Issues
A warning message appears after you install the desktop vSphere Client for Windows, and try to connect to an ESXi host
The desktop vSphere Client was deprecated in vSphere 6.5 and is no longer supported. When you try to connect to an ESXi 6.5 host, the following warning message appears: The required client support files need to be retrieved from the server "0.0.0.0" and installed. Click Run the installer or Save the installer.
In this case use the VMware Host Client to conduct host management operations.
André