In GC, you can only create a course from an activity already in GC or use the course creator.
What you tried should work in theory.
What was the source of the TCX file however? It would need to have meaningful timestamp as well and lon/lat data.
I converted your course TCX to an activity FIT file using the FIT File Repair Tool which you should be able to import into GC and create a course from with the turns added.
Yeah, good isn't it. I setup a course for an "exploring" run the other day and some might be interested in this pic https://www.instagram.com/p/BF_gCGouofB/ which gives an idea of the sort of display you get.