The option 'REMOVE SELECTED HOSTS' from a cluster is greyed out with the message "This functionality is not allowed at the moment. Please check back later."
VMware Cloud Foundation 5.x
This is a known issue that will be fixed in a future version of SDDC Manager.
Workaround:
Navigate to the Cluster view, wait for the Summary tab to load, and then navigate to the Hosts tab.
Note: Refreshing the page will retrigger the issue