Open an activity in Garmin Connect Web that contains the location of the segment you wish to create.
Scroll to the bottom, click on the Segment tab, then +Create Segment
Move the start/end slider to define the start/end of your segment
Click Save New Segment
If you're doing all that and still get Page Not Found, can you post the url of the page where you're getting that message? Possibly DNS issue or other web server related issue for your location
Had to remove the "segment/createFromActivity" part of the url because I don't think you can make the Create Segment link public. So I gather you get "page not found" every time you click +Create Segment no matter which activity you use.
Not sure what the problem is, if it is not the problem with Privacy Zones, but you can try trimming first the activity, reducing it just to the section you want to use as the segment, creating the segment from the trimmed activity, and then reversing back to the original (the trimming is reversible and repeatable as many times as you want). If you still get an error, try trimming it more. Perhaps there is some section of the track that causes problems, so in this way you could find it.
Do you also have your watch linked to strava? I read somewhere (on here, I think?) that if your watch is linked to strava you can't do a Garmin Segment. I had problems trying to do segments on Garmin, too, a while back and gave up in the end. Then I stumbled over the information, about Strava, somewhere, which answered my problem.
if your watch is linked to strava you can't do a Garmin Segmen
That's not quite correct. If your GC account is linked to Strava, you can choose between Garmin and Strava segments. You can do it in the GC Mobile app Menu » Training » Segments » menu » Garmin Segments or Strava Segments
For some reason, this option is not available in Garmin Connect Web, though.
