As Rebecca Long Bailey said for Labour during the debate on this amendment:
[N]ew clause 13 would require a comprehensive report into the UK tax gap, which is defined as the difference in any financial year between the amount of tax HMRC should be entitled to collect and the tax that it collects. Such difference derives from tax avoidance and evasion.
The amount that HMRC is entitled to collect is the legal amount of tax due and no more. As tax avoidance is legal it cannot be part of this tax gap therefore.