Hi. What you need to do is to first format tablet's firmware through SP Flash Tool by following the below steps:
- Click Format
- Make sure 'Auto format flash' is ticked
- Make sure 'Format flash (except bootloader)' is ticked
- Click Start
- Connect the tab while off with Vol Down held.
- Disconnect when you see 'Format OK'
And the flash the firmware as normal and all shall be well (remember to untick the preloader)