Why "upmost" doesn't reach the top

What kind of error is it?

Spelling and usage error

Why do people make this mistake?

"Upmost" sounds similar to "utmost", and people may mistakenly think they are interchangeable; "upmost" refers to the highest position, while "utmost" means greatest or most extreme in extent.

What is correct?

"Utmost" is the correct word to use when referring to something as being of the greatest degree or most extreme. "Upmost" is a less common variant and usually meant to refer to the highest position in a physical sense, but it is rarely used, leading to confusion.