bark up the wrong tree

To attempt or pursue a futile course of action, often by making some kind of suggestion or request. If you think I'll help you cheat, you're definitely barking up the wrong tree! I barked up the wrong tree when I applied to such good colleges with my average grades.

Fig. to make the wrong choice; to ask the wrong person; to follow the wrong course. (Alludes to a dog in pursuit of an animal, where the animal is in one tree and the dog is barking at another tree.) If you think I'm the guilty person, you're barking up the wrong tree. The hitters blamed the team's bad record on the pitchers, but they were barking up the wrong tree.

Waste one's efforts by pursuing the wrong thing or path, as in If you think I can come up with more money, you're barking up the wrong tree. This term comes from the nocturnal pursuit of raccoon-hunting with the aid of dogs. Occasionally a raccoon fools the dogs, which crowd around a tree, barking loudly, not realizing their quarry has taken a different route. [Early 1800s]

To misdirect one's energies or attention.

To waste one’s energy or efforts by pursuing the wrong scent or path. The term comes from the 1820s, when raccoon-hunting was a popular American pastime. Raccoons are nocturnal animals and generally are hunted on moonlit nights with the help of specially trained dogs. Sometimes, however, the dogs are fooled, and they crowd around a tree, barking loudly, in the mistaken belief that they have treed their quarry when it has actually taken a quite different route. “If you think to run a rig on me,” wrote T. C. Haliburton (a.k.a. Sam Slick), “you have barked up the wrong tree” (Human Nature, 1855). The cliché became especially common in detective stories in the 1940s, owing to the obvious analogy of hunter and hunted.