Hi mate,
You may want to check the log. I got the same message because the following reason:
Feb 13 08:36:15.559: Error: CLI 'interface output physical snmp' not valid in 'input' direction
Feb 13 08:36:36.160: Error: CLI 'interface input physical snmp' not valid in 'output' direction
After removing those commands from the flow record I was able to add the monitor to the interfaces.
Do you get something in your log when trying to apply the command?
I hope it helps.
Cheers!