If you are incompetent, you are not able to complete a task, like if you only ever made peanut butter and jelly sandwiches — you would be incompetent in the kitchen of a fancy French restaurant.
Someone who is not very good at getting a job done is incompetent. An incompetent salesperson can't sell anything. A learned professor may be quite incompetent when it comes to controlling a room full of rowdy pre-schoolers. Legally speaking, you can be declared incompetent if you do not have necessary qualifications, like being certified or licensed to do a certain job, like cut hair or fix cars.