Engaged in germany

Dear samunkhan173 thank you for reaching out to us.
Firstly, someone can apply for family reunification if one is counted as immediate family. This means this applies to married couples or underaged & unmarried children. Since you two are engaged and aren't officially married yet, unfortunately, you cannot apply for family reunion to her. You can read more about family reunification on Handbook Germany.
Additionally, since her passport stated her age as 15 years, she would need to provide proof that her biological age is actually 18 years. In Germany you cannot marry a minor person, which means that a visa to get married in Germany will be less likely to be granted as well. Please find more information on marriage.
I am sorry that I have no better news for now. It is very recommendable to seek advice from a migration counselling center which might have contact to some additional legal advise as well. There she can show all of her documents and discuss your options in detail.
Happy if you let me know her current place of living, so I can support finding a counselling center or she can look on this search mask from BAMF.
Please get back to me in case you have further questions.

Tnx dear she is living in wittenberg she is new there dont know anyone one to help here and to direct him so thank you dear sara if u could help us

Happy to help, samunkhan173
Did you mean Wittenberge in Brandenburg?
I found a migration counselling for your fiancee:
Stephanus-Stiftung
Migrationsberatung Prignitz
Bahnstraße 22
19322 Wittenberge/Prignitz
Tel. 03877 56 77 37 7
migration-beratung@stephanus.org
She can reach out to them and schedule an appointment. Please let me know if you have further questions.