Can I marry someone else if I came on a fiancé visa and it didn’t work 

Yes, you can marry someone else if the original K-1 relationship ended but you cannot get a green card through that new marriage while staying in the U.S., unless you qualify for a rare exception.

The K-1 visa only allows you to marry the person who petitioned for you and to apply for a green card based on that relationship.

Your new spouse (if a U.S. citizen) can file a new I-130 petition for you, but you will likely need to leave the U.S. and process the visa abroad.