This error message can occur when the memberships in question have more members than the membership level requires. 

Check the Number of Members allowed on the Membership Level​
  1. Click on the Memberships tab > Click on the name of the Membership Program
  2. Click on the Terms and Levels or Levels tab > Click edit
  3. Note the number of members allowed on the membership

Check the number of members on each relevant membership:
  1. Click Constituents > Click Constituent Search
  2. Search for and select the Constituents
  3. On the constituent's record click on the Memberships tab
  4. Click edit members next to the membership
  5. Click the gray square next to the extra member > press the delete button on your keyboard
  6. Click Yes, when asked if you want to delete the line > Click save
  7. Repeat Steps 1 - 6 for the other constituent that is involved in the Merge and any other memberships that have more members than allowed on the membership record
*Note: Alternatively you can increase the number of members on the membership to account for the number of members on the constituent's membership