If you attempt to create a checkpoint on a transparent block, for example GLASS, ICE, GLOWSTONE, STAIRS or ANVIL, the checkpoint is created in the config files, but the pressure plate isn't (can't be) placed on these blocks. So the resulting course has a checkpoint that can never be reached.

Some transparent blocks, for example FENCE and WALL will allow the pressure plate to be placed but don't trigger the checkpoint when you step on it during a game.

Creating a checkpoint on a TNT block will blow a crater in your nice new course.

Some transparent blocks work without problems such as REDSTONE_BLOCK, REDSTONE_ORE, SLIME_BLOCK, PISTON, FURNACE, SLAB (upper), STAIRS (inverted).

Some transparent blocks get replaced by the pressure plate but still function correctly as a checkpoint in-game, e.g. CHEST, CAKE, BED, RAIL, SLAB (lower) .