What are the duties of the coach in a football team?

Assuming you are asking about head coach as management team of football club can have other coaches like fitness coach, goalkeeper coach and like. See: Chelsea FC management team

Responsibilities of a head coach in a professional football club usually include (but are not limited to) the following:

1. Selecting the team of players for matches, and their formation.
2. Planning the strategy, and instructing the players on the pitch.
3. Motivating players before and during a match.
4. Scouting for young players for eventual training in the youth academy or the reserves, and encouraging their development and improvement.
5. Facing the media in pre-match and post-match interviews.
6. Delegating duties to other coaching and medical staff.*

*The management of the team (coaching and medical staff) might be under the club board.

For more information read this wikipedia article.

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<sub>References:
1. Manager (association football)
2. What is the difference between a manager and a coach in case of a soccer team?</sub>