Gmail Gives An Error When I Try to Add Alias Email

Gmail Gives An Error When I Try to Add Alias Email

My main email address with Zoho is working in Gmail - I can fetch and send in the Gmail client with POP setup.

However, I cannot add an alias email in Gmail.  Gmail give the error "
You already have an account with this server and username."  

How would I configure Gmail to receive/send from the alias?

Many thanks, 
Steve