When adding personal memberships via a Membership Dues Batch, users should only need to enter a value in the "Bill to" field. When a constituent pays for a membership for themselves, the "Primary member" field is implied, and so having a default in that field other than that causes this error - that's why the "Primary member" field is grayed out.
When adding a gift of membership, where one constituent pays for the membership of another constituent, both fields are available and can be populated with different values.
To resolve this issue, remove the default value for the "Primary member" in the batch template.
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