I have been comparing this tutorial with other ESRI learning source such as ESRI video (13:04). I found that this tutorial uses "Optional" for "Web_Map_as_JSON" in parameter setting of the script tool whereas the video uses "Required" for "Web_Map_as_JSON" parameter (please see screen shot below).
As ESRI's tip in the tutorial says that we can leave the input parameter for Web_Map_as_JSON blank, the script tool won't run successfully if I change the parameter to "Required".
I don't know if this difference is the cause of my problem but I have not been able to find any other potential cause yet. I appreciate If anyone points out a cause of my problem and give me some advice.
Thanks in advance.
I added this service URL ("https://MyServer:6443/arcgis/rest/services/ArcpyMapp/ExportWebMap/GPServer/ExportWebMap").
This URL is what I also used for the html. But once I type this URL in the *Service URL section, the error came across, "This URL is not available".
Thanks for the link, I will read through them.