Welcome to our community and thank you for your question!
Yes, in general it is possible for you to get married. Since you are a failed asylum seeker, the only way for you to marry is that your fiancé comes to Germany because you are not allowed to travel.
But you have to keep in mind that marriage in Germany is a quite complicated process that might take some time. You have to submit a number of documents at registry office - including your passport. That might lead to deportation, but it is inevitable if you want to marry. In addition, some documents might need to be sent to your home country in order to verify them. This procedure can take up to several months. But you run the risk of getting deported in the meanwhile.
So unfortunately there are a lot of obstacles to overcome :/