What can I do to remove this error of Specified account already being used as signal while It is not???
I would contact the Service Desk https://www.mql5.com/en/users/kanamua/servicedesk
They respond within a day, and I'm sure they could help you out.
Just to be sure, I would also check that you actually deleted your signal, and that it's not just disabled.
You cannot start a new signal with an account that has been used to share a signal again in the past.
I don't know, that the MQL5 rules.