Each eBay account must have a unique email address. eBay is telling you the email address you are trying to use is already associated with a different eBay account.

Try a new email address.

You most probably have another eBay account that you linked to that email address. If you don't use the other eBay account any more, you should log in to it, change the email address, then log in to your new main eBay account and link it to the email address you originally wanted to use.

When one is thwarted and stopped in there tracks,

by one avenue,

they are offered another fork to a solution.

Ie:New email address.