I would recommend upgrading to 10.8.2 at least. 10.6.x was retired earlier this year and is no longer supported: https://support.esri.com/en-us/products/arcmap/life-cycle
Do you have ArcMap signing into AGOL automatically? I get the same error pretty much anytime I try to use any AGOL data/basemaps/etc. in ArcMap.
If so, try signing out before running the tool(s).
R_