Are you upgrading a VMware vSphere ESXi host from 5.0 to 5.5 ? And after upgrade, are you trying add the VMware vSphere ESXi 5.5 to a VMware vCenter Server 5.0 ?
If the answer for the two questions is Yes, you will need to do what André said, upgrade the Vmware vCenter Server from 5.0 to 5.5 first, and then upgrade the VMware vSphere ESXi hosts from 5.0 to 5.5.