How can I change out an email access in our Notification recipient?

I have a Starter account.  I've been using this for about 3 or so years.  We had 7 recipients in the email section, and I wanted to swap a couple out.  After removing one, it seems like after removing one of the emails, I can't add another one.  I see a message that says "To use multiple recipients, please upgrade your account."

It seems like we were able to add the emails at some point, but now we're locked out of doing so.  

Can you help? 

I looked at the packages, and it's not clear to why there is a limit on the recipients list, or which package a person would need to get past this.

Unfortunately, you can only add one recipient to your form notification if your account is on a FREE plan. That restriction is stated in this guide: Send-Notifications-to-Multiple-Recipients

I'll leave it as it is. However, if you want to add another email address then you will need to upgrade your account.