>> If you are using Xiaomi/Redmi running MIUI 11 and later
Step 1: Go to Settings app > All specs > Find MIUI version and tap it for 7 times > Go back to the Settings screen and find Additional settings.

Step 2: Make sure the "Developer options" is turned on > Scroll down and turn on the "USB debugging" and "Install via USB" options as well.

>> If you are using Xiaomi/Redmi running MIUI 6 to MIUI 10
Step 1: Go to Settings app > Find About phone > Tap MIUI version/Android version for 7 times > Go back to the Settings screen and find Additional settings.

Step 2: Make sure the "Developer options" is turned on > Scroll down and turn on the "USB debugging" and "Install via USB" options as well.

>> If you are using Xiaomi/Redmi running MIUI 5 or earlier
Step 1: Go to Settings > About Phone > Tap on the Build number/MIUI version for 7 times to turn on the developer option.
Step 2: Go back to the Settings screen > Developer option, make sure the "Developer options" is turned on, then scroll down to enable the "USB debugging" and the "install via USB" buttons as well.