The net Vat due to be paid to HMRC (box 5) must be the difference between box 3 and box 4. The most common issue of box 5 being highlighted in red is where there is a penny difference on the source file.
The issue could also be caused by a formatting or rounding issue. To check this, please open your source spreadsheet and click File>Options>Advanced, scroll down to the section ‘when calculating this workbook’ and tick the box ‘set precision as displayed’. Once you have done this you will then need to check the formatting on each of the cells for boxes 1 to 9 to make sure that they are set as number and to 2 decimal places.
After any changes, we would recommend that you close and reopen the source spreadsheet and re-link the cells to VAT Filer so that they then update and try your submission again.