You registered the call yourself a long time ago
If you have signed up more than once for Club Log, you can easily arrive at a situation where you can no longer access one or more of your accounts - usually because you have forgotten the original login details. You cannot "take over" a callsign that Club Log believes belongs to another user so you must gain access to your old account in order to resolve this.
Use the password reset form to regain access to the old account, then go to the 'Settings' page and delete the account if you prefer to start with a fresh one, or update it if you plan to keep using the account.
If for some reason you still cannot regain access to your old account, please explain what has happened and submit a help desk ticket to delete the account so you can start over. Allow several days for this request to be processed, though.
Note: please do not register multiple accounts on Club Log unless you really need to. Club Log supports as many individual callsigns as you hold under a single login. Registering multiple accounts can cause confusion.
Someone else has registered the call (eg. reissued callsign, or other mistakes)
Club Log has an automated system to allow callsigns to be re-used. This happens with reissued callsigns, special event callsigns and expedition callsigns reasonably frequently.
You can use the Callsign Transfer Request form to ask for a callsign to be transferred to your account. The request form is here:
https://clublog.org/requesttransfer.php
It will take up to 28 days to resolve, depending on how quickly the current holder responds.