hi,
i Understand what you are dealing with, but this is one of the down sides of playing on a non-premium Minecraft Account, as changing Names means you are on another account, in your Case it seems you're trying to log in With a Username of a Premium Account that has already played on JartexNetwork Before, so it won't let you do so. it is kind of a Protection similar to the
/login (password) for non premium accounts.
You also might wanna open a Support ticket instead of a General Thread, or just ask other people on the #Assistance-chat channel on JartexNetwork official discord!
I Hope i helped you a bit, and Have a good day!