Is refraining from informing the neighbors about the action of the neighbor who blocked the sewage, out of fear of problems, considered a sin and forbidden?

If a neighbor is doing something that harms their neighbors, silence is not permissible. Advising Muslims is an obligation for those who are able, and it involves guiding them to what benefits them in their afterlife and their worldly life, preventing harm from reaching them, enjoining them to good, and forbidding them from evil with gentleness and sincerity.

If advice does not work, or if harm is feared, then it is obligatory to inform someone who can prevent this injustice. This can be done without identifying the person reporting it, such as sending an anonymous message.