Thank you. With what partner are you attempting to add the zone?
The error indicates either you or someone else added the zone to Cloudflare either via a partner or directly. If the domain is your and if that was not you adding the zone or was you and the zone is active in an account you can no longer access/do not recall, the only option is to add the zone to Cloudflare and move it to an account you can access/recall.
If you did not add the zone via the partner, I suspect the route is:
As long as you can instruct your domain registrar to change the name servers to those assigned by Cloudflare, I’d add the zone directly to your account, change the name servers. Once the zone is authenticated and the name server change is confirmed, you have control. From there, you can decide if you want to continue directly with Cloudflare or sign up via a partner.