This error comes from one or more of the EtherCAT slave devices that are configured in the System Definition not being correctly visible to the master and thus unable to be communicated with.
Power-cycle all slaves and ensure that all of the EtherCAT slaves are correctly communicating.
If all slaves appear to be communicating properly, create a LabVIEW project targeting the Real-Time target with EtherCAT slaves. Deploy the EtherCAT master and then try to transition each slave to Pre-Op individually using the "Online Device State" property window accessible by right-clicking the slave devices. You will find that one slave does not transition or appears disconnected.
Note: With Scan Engine in configuration mode, you can only transition between Init->Pre-Op. To enter Safe-Op and Operational mode, change the Scan Engine to "Active", which will transition all slaves at once.
If this does not resolve the issue, please contact NI Technical Support