=> For Enabling to developer Mode In Redmi Note 3 :
1: Go to Setting.
2: Click to “About phone” option.
3: Press 7 times to “MIUI version” option.
now you will see a Toast message “now you are developer”.
=> Now you should install to Mi PC Suite from this .
1: Install the pc suite with connected your redmi.
2: Now go on Permission in setting and click “install via USB”.
=>For Enabling USB Debugging :
1: Open Settings > Additional settings > Developer options.
2: Firstly enable to developer options then go below and find USB Debugging and enable it.
3: When your pc suite open then again repeat disable and enable to USB Debugging
For Redmi 4a also follow :
Open Settings > Additional settings > Developer options > Select USB Configuration > MTP.