Thank you for your question. It is not obligatory to do an istikhara for marriage and if it comes out negative there is no obligation to act in accordance with it. A person should not take an istikhara unless they are unsure and do not have a preference for a certain outcome.
However, the spirit of istikhara is to seek the best from Allah, so if all of its conditions have been fulfilled the answer a person has been seeking is within the istikhara.
May you always be successful