Hi Sam,
In Smartzone 5.0, Depending on where you are attempting to start capture mode, you may need to provide the MAC address of the AP and DataPlane (DP). If you highlight the AP in question from the Access Point list, hit the "More" drop down and begin the .pcap from there, you should be presented with options for 2.4GHz/5GHz/Wired interface, and whether to save as a local file or stream to a Wireshark endpoint.
If you are attempting to capture from a radio interface you have turned off (if you've disabled 2.4GHz in your environment for example) it may throw the error you are describing.