My parents want me to marry a person. They want us to get to know each other and after a month I yes I want him or I don't want him.But if I say no I need a valid reason that can't be fixed.What should I do?

Marriage is a very important relationship, it is taking your love relation to next level, where you promise to live with that person and love them, care about them and support them. If you don't want someone, those promises are void and neither you nor that person will be happy. If I were you, I would meet that person and try to know that person but I feel 1 month is sufficient to realise if that person is meant for you. Ask them for more time and still if you don't want that guy, tell your parents you can't marry as you can't spend whole life in an unsuccessful marriage and tell them at the end of the day they and you both want the same thing that you meet right person and stay happy with each other. Your happiness matters the most, if you don't feel happy tell them as we both know that you and you parents want you to be happy.