Try the “correct distance” magic: open the activity on the Strava website (not the mobile app); then in the menu on the left, click the three dots, and then Correct Distance. This will trigger recalculating of the run based on the map data, and likely will get you over the 21.1 milestone. Note, it might take a bit for the change to take effect. (If you're not happy with the outcome, it's possible to revert the change.)

@Jana_S I tried this and it went to 21.01 km so reverted back 

Oh, sorry to hear that So hopefully you'll hear from Support soon!

It’s possible to course wasn’t the exact distance, as well as going for example to the inside of a corner will shorten it, but on top of that, is GPS accuracy. You said Strava logged it, so I am guessing you used your phone?  I have found my phone accuracy isn’t the best, but will run an app called GPS connected that keeps the GPS active and helps with accuracy… That doesn’t help you now though, the GPS data recorded is what was recorded.

However, you can manually enter your PR for a half-marathon with the chip-time or race time provided in the race results.

On your profile page, (web version), look at the stats on the right side, make sure it is showing running stats. Under Last 4 Weeks and above Best Efforts, you can see All-Time PRs. Click Add PR to the right of that, then select half-marathon, and enter your time. It will ask for your race time, your race results link, and the Strava activity link.

It could be a combination of things:
 - GPS interference along the course caused by large obstructions (buildings, trees, etc.)
 - The course may have been just a little bit short (but that can be confirmed through the race organizers)
 - The accuracy recording device may have shorted the distance

 - STRAVA tax

Hoping you get it resolved!