It refuses too chamfer because it's "self intersecting"

cory_turton191

https://cad.onshape.com/documents/4a97a26e472106d38be181d7/w/028817140a1975a3a5dd0d7d/e/1ff8e026d4d9521cca36e2ea

dirk_van_der_vaart

I can,t see what the problem is because you need to share your document first.
But in the view I can see that you just started with your design, and may be there is something wrong with one of these sketches.
And another tip don,t use chamfer for designing, if you want it in the design put it in your sketch