Gear making noise and fuss

I brough a car there last year November 22- 2025, the was 6400km and still in good condition so fresh but while i was driving i experience gear making noise especially reserve and other gears are fuss while driving..i booked for check up January and February took for service they told me i must pay R1100 for dignose and R13500 for gear repair..they never explain anything about the problem they said it was once fixed the same issue and warranty is not covering the mechanical issue ..within 6 months they claim i could be driving pattarn i asked them how could i damage a clutch gear within a month been driving so now i must pay please help hellopeter