Cannot add a domain because it says it's already associated with an account

Cannot add a domain because it says it's already associated with an account

I am helping someone with a domain that I cannot add to Zoho because it says it is associated with my account on Zoho. I must have accidentally added that domain to my account, but I cannot find it associated with mine. So, I need help removing that domain wherever it is, so that we can create its own account. How would we do that.