Hi Daniele,
Sync RPO violation status means that during sync operation is in progress VR hasn't reached the defined RPO (in your case it is 3 hours). Several reasons could lead to this state - network connectivity between the sites, network issues with the host and/or datastore, a lot of changes need to be sync but your network is overloaded , etc.
In order to recover a VM replicated with VR it is required to have the initial full sync completed to the target site, e.g. in the VR UI the last completion time, duration and columns must be filled with the correct values. The recovery operation exposed as a button over the replication list is enabled in the cases when you are not allowed to recover - such as recovering is already in progress, moving the replication (if you are using VR 5.5) from one replication server to another, still in configuring state, etc.
Hope this helps.
Regards,
--Martin