If I am a solo traveller, but book a trek to a region that requires a minimum of two people, what happens if I don't fnd another person? Can I still go since I paid the extra permit fee? Or is it not possible without a second person?

Yes, it is still possible but you need to pay for two permits which will be charged to you if you are only a single person on a trek that requires two people. Once a payment is made and a trek is confirmed it will go unless you cancel it.