Hiya, CSV,
Let us know what tablet you have and what version of Android you're using. It might be that you haven't enabled developer options, yet.
Go to menu>settings and scroll down to near the bottom. If you don't see "{} Developer Options" the tap on "About Phone", scroll to "build Number" and tap that 7 times to enable developer options. Then you should be able to turn on USB debugging.
Also, try enabling it without the tablet plugged in.