Mental Set
Mental set is the tendency to approach a problem the same way you approached similar ones, even when that way no longer works.
Why it traps you
Your previous solution becomes a rut, and the alternative becomes invisible.
Functional fixedness is a specific case: difficulty seeing new uses for familiar objects. A box is for holding things, so you don’t think to flip it over and use it as a stand.
Mental set is the main reason smart people get stuck on problems that look easy in hindsight.