Valid non-occluding materials for checkpoints are limited to slabs in https://github.com/A5H73Y/Parkour/blob/master/src/main/java/me/A5H73Y/parkour/utilities/Utils.java#L924.

It would be nice if glass blocks (of all colors) are added to this list.

Or even better: make this list adjustable by config.

Improved considerably in v6.7 release, no longer an issue.

Prior to version 1.14, pressure plates could not be placed on glass blocks, which is an example of why that code tries to filter out invalid blocks for checkpoint placement.

But things change in each Minecraft version and as we are trying to support servers running 1.8 to 1.16, its not possible to code for every eventuality, so there is a config option you can use to override the safe checkpoint checks :

Other:
  EnforceSafeCheckpoints: false

That will allow checkpoints to be created pretty much anywhere.