Is one sinful if he sees an evil and does not denounce it due to his inability to do so, because of a psychological complex or fear? And what do you advise someone who suffers from this?

Enjoining good and forbidding evil is one of the obligatory duties of Islam. It is a communal obligation (fard kifaya), meaning that if some Muslims undertake it, the sin is lifted from the rest. If a Muslim is unable to perform it in any way, the obligation falls away from them, and it suffices for them to denounce it in their heart, which is the weakest form of faith. So, whoever fears real harm should suffice with denouncing it in their heart, and by doing so, the sin of remaining silent about the evil is lifted from them. This denunciation, however, never falls away. Whoever dislikes evil in their heart is absolved, and whoever is pleased with it is sinful.