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I'm trying to change ownership to my main brand account but am getting this error.
I'm not sure what the issue is since both emails are verified and the other email is already an Admin. How to fix this?
This issue could happen if the account that you have signed up for by the new email ( say firstname.lastname@example.org ) has not been claimed yet. That's the reason the change ownership operation throws an error.
You could go to the app.chargebee.com link and give the email ID - email@example.com and you'll be prompted to claim your account and an email will be sent to your inbox; You can read more on this here.
Then you could claim that account with a dummy name and only after claiming the site as mentioned above, you will be able to change the ownership of the Chargebee account.
Related articles and Documentation
Can the Chargebee account ownership/email id attached be changed?
Chargebee Signup, Email Verification & Account Claim Process.