When you use the expression ‘Can’t Cut the Mustard’ you mean that someone is unable to succeed or meet expectations.
Example of use: “I really like Jake, but he just can’t cut the mustard.”
The phrase ‘can’t cut the mustard emerged in America at the end of the 1800s. The earliest printed example comes from an 1889 edition of The Ottowa Herald: “He tried to run the post office business under Cleveland’s administration, but “couldn’t cut the mustard.” The use of quotation marks indicates that the phrase was already a common one.