UserName you entered is already exist

UserName you entered is already exist

I am trying to integrate Zoho Campaigns with Zoho CRM. I used a different email address when I set up Zoho CRM, but I have changed this to the current email address.

I can see the old emails when I select integrations on Zoho Campaigns, but when I try to add my new email address with the ZSC, I get the message ' U serName you entered is already exist' even though the email address is not displayed.

How can I integrate Zoho Campaigns with Zoho CRM?