You have to prove it is the employees fault by intentionally not completing the task. Incompetence or incapability is not the employees fault, because well, you hired them and judged their ability - probably due to their education (it is a little more complex than that of course). As a concrete example, if you have a CS degree from the 80s and job as a COBOL programer and your employer decides to assign you to a new team doing react+js, the employee is still formally qualified. You couldn't fire him for incompetence, just because a 22 year old bootcamp graduate delivers 10x the results.