Hi Iam Jin-tae Kim, an South Korea ArcGIS User.
When I Printing a WebMap in ArcMap 10.6.1, I met an Error Message and Print Failed.
ERROR 000800: The value is not a member of MAP_ONLY
I found This Error Message on ArcGIS Server Manager Log.

Before I did this, I changed the Protocol on ArcGIS Server to HTTP Only -> HTTP & HTTPS, and I also added HTTPS for the web adapter.