Cloud functions can be activated provided that you have at least three cluster nodes, however, the fault-tolerance features will work only on 4 and more cluster nodes and a network storage configured.
More information about network storages can be found under How to connect storages.
To enable cloud functions, navigate to the Cluster settings → Cloud functions:
Enabling cloud functions will start corosync and corolistener on all of the cluster nodes. In the list of cluster nodes in VMmanager you will see the icon indicating that the node is included into the cluster. The /tmp/.lock.vmmgr.firstinit and /tmp/.lock.vmmgr.service files are created on the master node. The system starts the database replication and enables the needed systems (corosync, corolistener). The /tmp/.lock.vmmgr.firstinit file is deleted.