The phrase "fruitless effort" means that someone's hard work or attempt to achieve something was unsuccessful and did not yield any positive outcomes. Full definition
Hmmm... I never took you to be a masochist or one to partake in fruitless efforts. (sbnation.com)
Most of them are men, and after much fruitless effort, she gets an idea. (law.stackexchange.com)
After several fruitless efforts to restrain the Donatists, Constantine in 321 granted them full liberty of worship, though this toleration was denied by his successors. (firstthings.com)