This error message will occur when the number of members on the membership of either the source or target constituent exceeds the amount of allowed members for that particular membership level. You can resolve this issue by reducing the number of members on the constituent's membership to be equal to or less than the number of allowed members on the membership level

Check the number of allowed members on the membership level

1. Click on the Membership tab > click on the name of the membership program
2. Click on the Terms and levels or Levels tab > click edit
3. Find the membership level in question and review the number of members allowed
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Check the Membership of the two constituents and remove any additional members that exceed the amount of members allowed from the previous steps

1. Click Constituents > Constituent Search
2. Search for and select the Constituent
3. Click on the Memberships tab on the constituent's record
4. Click Edit members
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5. Click on the name of the member > click the remove button in the upper right hand corner of the display box
6. Repeat step 5 until the number of members are equal to or less than the number of allowed members for the membership
7. Click save
8. Attempt to merge the constituents again