Bad credit score

At fist when u joint them they were great had no problems got my certificate. However when I joint them back in September 2022 had to cancel due to financial reasons they send a e-mail to confirm the account was close and I don't owe them However in January-April 2023 they show on my credit file as misses payments but no amount was deducted because I canceled a month in advance to cancel ..so I asked to send me a letter to state I owe them nothing they refuse to send I even called then they hang up on me ..so now I have a bad credit sore because of them for just requesting to further my studies with them so disappointing with them will never make use of them ever again