Is it permissible to complete a project for an advertising website for wedding necessities (wedding halls, hairdressers, car decorations) after paying a portion of the amount, or is this unlawful?

There is no harm in creating a website to advertise shops that sell wedding supplies, unless the shop predominantly deals in unlawful (haram) things, in which case it is not permissible to advertise it, due to the prohibition of aiding in sin and transgression.