Could not convert variant of type (Integer) into type (Dispatch)

Hello,

From your screen shot the value "--inPrivate" has two dashes and the command correct line "start msedge.exe -inprivate" to start Edge in private mode is with a single dash.

As per TestComplete documentation https://support.smartbear.com/testcomplete/docs/app-testing/web/general/common-tasks/launching-browsers.html#Arguments you are setting it correct, and I failed to make it work as you are experiencing. Probably create a support incident reporting it.

For a work around, use "Rub Code Snippet" rather than "Call Object Method".