This constituent or a related record (Household, Spouse, etc.) may already have an existing membership, and your membership program may not allow multiple memberships to be held. Try any of the following solutions to resolve the issue.

Solution #1
Check the Memberships tab of the constituent record, their Spouse's record and their Household record, and ensure that any current membership record in that same program is Cancelled. Any other status (such as Lapsed, Expired or Grace) will result in the error message when you try to renew the membership. If you need to cancel the membership, click on Other Tasks and then Cancel the Membership.

Solution #2
Renew/upgrade/downgrade the existing membership instead of adding a new one.

Solution #3
Temporarily edit the membership program to Allow Multiple Memberships. Here are steps:
  1. In Memberships, select the hyperlinked membership program name
  2. On the General tab, click Edit
  3. Mark the checkbox to Allow multiple memberships
  4. Add the constituent to the membership
  5. Repeat steps 1-3 to unmark the Allow multiple memberships checkbox

Solution #4

Create a duplicate constituent record and sell the membership to this new record. Then, users will be able to merge the duplicate into the original (target) constituent record, and this will combine the membership data into one record.