Sometimes when using the "Run Application" action in combination with storing the Main Window handle into a variable the action may fail with the error "Cannot retrieve Main Window Handle". It has been noticed that depending on the system and application used, sometimes the handle might not be retrievable under certain circumstances.
In case you receive this error, (or planning to run the process on another or on multiple machines), it is suggested to leave the action's "store Main Window handle" field empty.
The option to store the Main Window's handle is available when the "Wait for Application to Load" property has been selected. For more information you can also read Identify a window by its Handle