Can you show the output of the "$protectionGroups[0] | gm"command? It looks like the ListAssociatedVms() method needs a parameter. In the
"Site Recovery Manager API Developer’s Guide, VMware vCenter Site Recovery Manager 5.0" (https://www.vmware.com/support/developer/srm-api/srm_50_api.pdf) a "managed object reference to an SrmProtectionGroup object" is used as a parameter.