wild-goose chase

noun1

1. •
   (figurative) A futile search, a fruitless errand; a useless and often lengthy task whose execution is inordinately complex relative to the value of the outcome.

   • I went on a wild-goose chase all over the town looking for that adapter until I discovered they no longer make them.
   • Diagnosing this software application's problems is a wild-goose chase because it is built in an environment that has poor debugging tools.