Deleted member issue

I am trying to turn over an account I created for a client to the client. Currently the Super Admin email address is a shared email I created for the client and I want to that to now be the client’s direct email. Problem is that email was on the account as an additional member so was already in use. So I deleted that member and then proceeded to try to change the Super Admin to the client’s email address and I am still getting the “email in use” error. Does anybody know how to fix this?