When running "arcpy.SummarizeWithin_analysis()" in PyCharm, I get an error saying the module isn't available. I've done "import arcpy" and I have ArcGIS Desktop 10.6 Advanced. It also doesn't show up as a tool in my Statistics toolbox in ArcMap. According to the webpage, it should be available for my license: http://pro.arcgis.com/en/pro-app/tool-reference/analysis/summarize-within.htm#L_
This tool is only available in ArcGIS Pro.
If you look at the Analysis Toolbox, and Statistics toolset inside ArcMap, you'll see that Summarize Within is not there: http://desktop.arcgis.com/en/arcmap/10.3/tools/analysis-toolbox/an-overview-of-the-statistics-toolset.htm
Also see the History section of the Analysis Toolbox inside ArcGIS Pro: http://pro.arcgis.com/en/pro-app/tool-reference/analysis/analysis-toolbox-history-pro.htm It states that Summarize Within is "new to Pro 1.0". That is, it was not ported from a version of ArcMap (like you can see for other tools)
Note - this tool is available at all license levels.