climb the stair? character keeping fall over.. how can i stop it?

as iam working on my 3d worlds. iam stuck with that small things again.
as ive try to walk over hill with plane hidden below it. its okay. still working. but when i try walk up the stair. i keeping fall over and unable to stand up again or unable to climb up the stair…

so. how can i stop character slid when climb up/down the stair / ladder?

Note - iam artist so i has no skill in programmer. only knew a tiny bit.