What is the legal ruling on an application that offers daily tasks in exchange for a fixed profit after 75 days, with no possibility of recovering the original capital, and is this considered usury (riba)?

Your subscription to an application by paying an amount of money in exchange for earning from watching advertisements is impermissible, as it is closer to gambling.

Earning from watching advertisements—even if you don't pay anything—is not free from prohibitions, including: - Many advertisements contain impermissible (haram) elements. - These advertisements are for entities that pay money in exchange for their display, and websites hire people to open the advertisements to delude the advertisers that they have been viewed thousands of times, which is deception, fraud, and cooperation in sin.

The ways of earning halal income are many, for "Allah's bounty is only attained through obedience to Him."