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
Ensure that there are not any pending membership sales orders for this constituent under other users. If so, each of these users will need to cancel their pending sale.
  1. From Analysis, Click Information Library, Click Add an Ad-hoc Query > Choose the Source View of Sales Order
  2. On the left, Click Sales Order. From the middle drag Sales Order Status to Include records where and set equal to Pending
  3. From the middle drag Sales Method Type to Include records where and set Not Equal to Group Sales
  4. From the middle drag Sales Method Type to Results fields to display
  5. From the middle drag Added by User Name to Results fields to display
  6. On the left, Click Constituent. From the middle drag Name to Results fields to display 
    User-added image
If there are any users with pending orders for the constituent that is encountering the error, please reach out or log in as that user. Navigate to the sales method of the pending order and click Cancel. 

Solution #4
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 #5
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.