An oVirt data center is like a physical data center. It has multiple servers and various storage and network options. Every oVirt data center needs a cluster, a data storage domain, and a logical network named ovirtmgmt.
The purpose of an oVirt cluster is to group its hypervisor or oVirt nodes. A data center can have multiple clusters. In general, hypervisors in an oVirt cluster have same CPUs and the same network configurations. Another name for an oVirt cluster is migration domain, as you can only live migrate virtual machines within hypervisors in a cluster.
Every hypervisor in an oVirt data center should have access to the storage domain created for the data center. If you cannot give access for storage to any of the hypervisors, then you have to create a new data center and cluster for them.