This should be posted in the 'How Do I' section. Maybe ask a Mod to move this.
Anyway, using search, I found this thread with an example of a slippery surface. Hope it helps.
Also, You are using a dummy sprite - cPlayer - and setting sPlayer (the one with the animations) to it every tick, which is good, EXCEPT you have given them both the Platform behavior, which defeats the object and stops your code from working. The sprite sPlayer should NOT have the Platform behavior, which means re-coding some events.
As an example, Level1Events, Event number 21 checks for cPlayer on a slope, but then tries to simulate Right movement using sPlayer in Event 23. If this is set to cPlayer instead, your character slides.
Anyway, hope this is enough for you to go on with.