First things first: your units are in cm and tolerances are at 0,01 units. One of the short edges is 0,39 units - you are working too close to the tolerances. Best to change units to mm and better also to change tolerances to 0,001.
First make sure you get a single object by deleting, trimming and joining surfaces.
You didn’t mention radius or so so the attached is just to get you on the road.
I used the FilletEdge command and for the uprights, I added handles to allow the fillet to go to 0. I put in one fillet that goes to 0 at both ends and one that only does that on one end.
If you don’t want variable radius fillets, see the approach on the same shape that is copied behind the original one. First perform the fillet, make sure to cap holes, and then cut with the sloping surface.
FilletProblem-wd.3dm (911.2 KB)