v^2 = u^2 + 2as, is an equation suitable for constant acceleration. In this scenario the acceleration cannot be assumed to be constant, so this formula is not appropriate. The student is not able to find the value for either ‘a’ or ‘s’ to use in this formula.
In my opinion v^2 = u^2 + 2as, is a manifestation of conservation of energy, only when the acceleration is constant.
I hope that this goes some of the way towards explaining what the exam report is saying.