When I toggle on the install via USB in developer mode:
I had this issue on my Redmi Note 4 with MIUI 9. For me the procedure is very simple.
1. Go to Settings -> Permissions -> Gear icon at top right corner.
2. Give the required permission for Install via USB.
3. Goto Developer Options and try using Install via USB. It will work now.
This process is for one time. I hope you have resolved this issue too.