Why can't engines solve certain positions?
Engines are not gods. They are just extremely powerful, which does not mean they have no weaknesses, let alone that they can solve any position; misjudgments are not rare.
The most common case is composed endgame puzzles (a 排局 is a set-up position; most have few pieces and are often called endgames, and humans are usually expected to master them). In some puzzles where one side wins by force, it is extremely hard for an engine to find the correct solution.
So there can be certain positions in which humans are stronger than engines. By analogy: it is like a race between a human and a car. On the vast majority of tracks the car is stronger, but on some tracks there is a low wall, and there the human has the advantage.
Puzzles that engines find extremely hard to compute are the more memorable ones, because humans can master them and know their results. Another type of position is often overlooked, yet engines may also find it extremely hard to compute: advantageous openings.
Engine tests often use advantageous openings, for example Red high-advantage openings, where two engines each play Red and Black once. A result of one win and one draw occurs often. The engines may be unable to reliably draw or reliably win these positions, so they can also count as positions the engine "cannot solve", and humans are even less likely to know whether these advantageous positions are actually wins or draws.
Also, the only standard for an engine's playing strength is scientific test data; comparing only some positions cannot represent an engine's strength. Pikafish strives for higher playing strength and does not treat unsolvable positions specially; everything is based on strength, and most methods for solving special positions would cost overall strength.