There could be several reasons.
Hope my suggestions help:
Does the seller accept returns? If it is a private seller and you are returning for a change of mind reason, the return may not be accepted.
Do you need to add a photo? If you have not completed every section of the return, then it won't send.
Is it a valid tracking number? I have sometimes tried to put a zero when it is a letter O.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)