incompetence
A lack of skill or ability to do something correctly.
Incompetence means lacking the skill, knowledge, or ability to do something properly. When someone shows incompetence, they repeatedly fail at tasks they're supposed to handle because they don't understand what they're doing or haven't developed the necessary abilities.
A babysitter who leaves young children unsupervised shows incompetence because they're failing at the basic responsibility of the job. A surgeon who hasn't learned proper techniques would be dangerously incompetent. An incompetent soccer coach might not know the rules well enough to teach players correctly.
Incompetence is different from making occasional mistakes, which everyone does. It means a pattern of failure that comes from not having the right skills for the job. Someone might be highly competent at playing piano but incompetent at fixing computers.
The word incompetent describes someone showing this lack of ability. You might hear people say a repair person was incompetent if they made the problem worse instead of fixing it. Organizations sometimes fire incompetent employees who can't perform their basic duties even after training and practice.
Notice that incompetence isn't about intelligence. Smart people can be incompetent at tasks they haven't learned or practiced. A chess champion might be completely incompetent at speaking French if they haven't studied the language.