Thanks! this works.
Just to clarify, courses are found under training
connect.garmin.com > Training > Courses
To follow up on this, I used the add timstamp tool here: https://gotoes.org/strava/index.php to create a new GPX file, which I was able to import to Garmin Connect.
There must be a bug in the garmin connect web. Cause I get the same error. Honestly I can't believe in 2019 all this mapping creating - exporting - importing and what not is so complicated.
I found a much easier way to deal with this. Create your GPX route on your desired map on your PC. Export it onto a cloud drive ie. Google Drive.
Open Google Drive on your phone. Find your GPX file. Select open with... and Garmin connect will automatically detect it, Select the course type in Garmin connect app. Let Garmin connect generate the route and then click send to device. Voilá. It's easier that way. No need to connect with USB cable and all that complicated stuff.
Thanks for that! I've been messing around importing using conversion sites so this removes all that faff!