Unable to connect galaxy tab pro SM-T520 to PC with windows 8.1
You need to enable USB debugging on the tablet, and then install the USB drivers on Windows. For explicit instructions, see steps 2.1 and 2.2 in the following guide:

How do I install CyanogenMod on the Samsung Galaxy Tab Pro 10.1 Wi-Fi?

Again, you don't have to follow the whole guide to install CyanogenMod, just steps 2.1 and 2.2.