Error: The monitor factor of PROFINET device '...' must be set to 3 when operating in RT class IRT.
Set the parameter 'Monitor factor' of the device mentioned in the error message to '3'. This has to be done in the 'Interface' editor of the device.
To open this editor, double-click the error message. Alternatively, you can double-click the corresponding node in the PLANT.
Example:
For more information refer to the tooltips in the 'Settings' editor of the Interface by hovering the mouse pointer on the 
symbol.