This is a pesky common error that has plagued vSphere administrators for a long time. If you do not have multiple redundant datastores, you will get this error.
The number of vSphere HA heartbeat datastores for host is 1, which is less than required 2
To fix this issue for vCenter Server Appliance 6.5, follow these simple steps.
Login to vSphere Server Appliance web host client. Go to Hosts and Clusters from the main menu. Select the cluster you want to modify from the tree.
Go to Configure tab and select vSphere Availability (used to be called vSphere HA), click Edit
Go to Advanced Options, click in the Options box and paste this:
Paste this in the value box: true
Click OK, wait a bit for it to modify all your hosts. Done!