Nimble group harass you an track you but when you fully paid up not reachable.

I had a loan with African bank which after much debate and calls African bank established that my loan is indeed paid up. However they can't give me a paid up letter. Nimble group isn't even reachable. Why is it so difficult to get a paid up letter ? I don't owe you guys anymore but still you don't want to have me proof of this . Who do I go to for help now???