Reims is a well-known city on the Canal de l'Aisne à la Marne. It's hard to believe that canalplan doesn't know about it. How can one add a place? (I have looked but can't see a way). Actually, the canal isn't in canalplan either...
Martin/
The canal is there :
https://canalplan.org.uk/waterway/gxu8
It just seems to be missing everything apart from the locks and the tunnel.
There are plenty of help pages or you can ask. You can find all canals in the Gazetteer. The reason Reims is not shown as nobody has added it. Feel free to put as many places on the canal as you want.