Quranism is the position that the Quran alone is sufficient, without the Sunnah or Ijma.
The Quran gives precise shares for distributing inheritance.
Let us take a concrete case: a man leaves behind a wife, two daughters, a father and a mother.
The Quranic shares:
Total: 1/8 + 2/3 + 1/6 + 1/6 = 27/24.
You cannot give 27 slices from a cake that only has 24.
The Quran alone does not resolve this case.
The solution Al-Awl, a proportional reduction, comes from the Sunnah and Ijma.
Without the Sunnah and Ijma, this case is mathematically unsolvable.
If Quranism is true, Islam is false.