Subscribers is invalid error message:

This issue is usually caused by having multiple members in your member list sharing the same e-mail.  Our system should not allow this, but on rare occasions it does occur.  So to solve this problem remove one of the duplicate memberships - a duplicate can be identified by: 

• Review your members list via your team's website dashboard / members. If you have a small list it may be easy to identify a duplication.
• If you have a large member's list you can export your list via your dashboard / members / export / download CSV.  Then place your members in order via the e-mail column on the CSV file to enable you to identify a duplication.
• Once identified, remove any duplications to resolve the problem.