What if the process mentioned above doesn't work? I have tried holding down the volume button and pressing the power button, it's not working on my Samsung Galaxy Tab E. Am I doing something wrong?The short answer is buy a new one because they don't lock by themselves. If you bought it on e-Bay, send it back.
If you want to keep it, hold down the Volume Down button and press the power button. Select Factory Reset by using the volume button to navigate.
When it completes the process, it may still demand the correct G-mail address and password to get into the system so it will be quite useful - as a brick.