Not entirely sure , what sign in problem you mean but I think you are referring to the Google Account Verification after factory reset on Lenovo TAB-3 7 Essential . To fix this issue you will need to do a "Hard Reset"
How to Hard Reset Lenovo Tab-3 7 Essential
- Hold down the Power key for a short while in order to turn off the cell phone.
- Press and hold the Volume Up + Power button simultaneously for about 10 seconds.
- In order to choose "Clear eMMC" use the Volume Down to navigate and the Power key to confirm.
- After the Hard Reset you will not get any account sign in problem.
Hope this will help , if it doesn't we can try other methods aswell