To correct this error, you have two options: 

Option 1: Increase the number of cards allowed on the membership level before merging
  1. Go to Memberships and click on the hyperlinked name of your membership program
  2. Go to the Levels or Terms and Levels Tab
  3. Click Edit to Edit the Levels
  4. Change the number of cards allowed to the number that exists on the duplicate record.
  5. Click Save
  6. Process the merge again.

Option 2: Delete one of the member cards on the membership so that it does not exceed the number of cards allowed.
  1. Go to the Constituent Record
  2. Go to the Member Tab
  3. Click on the hyperlinked name of the membership program to go to the constituent's membership record
  4. Under Member Cards, click the drop down arrow beside one of the cards. Click Delete.
  5. Process the merge again.