I just want something new, what to write on resignation latter?

I've only been on the company for 4 months and I already want to quit, the reason is that there is no teamwork and everyone from my team just go on it's own, thinking that they are so good and that they don't need help. But I don't want to write that on my resignation letter since the render period is 60 days and I don't want any conflict. So I'm thinking "I just want a fresh start next year" is that bad? what should I write instead?

Also we sign a yearly contract (for evaluation), but its not stated on the contract that I have to work for 12 months once it's signed? so am I allowed to resign?

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