When two things (or people) don't work well together or correspond with each other, it's a mismatch. If there's a mismatch between the amount of free time you have and the hours of exercise your puppy needs, you might want to hire a dog walker.
There might be a mismatch between workers' skills and the actual jobs that are available, or a mismatch between your aunt and uncle, who seem totally unsuited for each other. Mismatch is also a verb that means "match up badly," like when you mismatch your red shirt with your purple pants. Sometimes sports matches or games are called mismatches too, when one team is clearly superior to the other.