This could happen because Collector was not installed or if it couldn't connect to the Management Service. Veeam Virtualization Extensions Management Service should have healthy collectors to be able to add Vmware Server into monitoring, without collectors it simply cannot collect anything from vSphere Infrastructure.
Did you use Manual Collectors deployment during the setup or automatic deployment? If you selected Autodeployment feature, you should add at least one Management Server into the corresponding Operations Manger Resource Pool and then wait a little (could take a couple of hours - restart Health Service on the target Management Servers and then on the VES Management Server to force the deployment).
When Collector is installed and registered (you can check on the Veeam Collectors Tab), Management Service can assign collection jobs and Add VMware Server button should be enabled.
Let me know if you have any other questions.