You may be entering membership dues in the batch form view, and want to add a promotion. You hit the promotion button, from the dues tab, and receive the error: "The given key was not present in the dictionary. You may be able to enter the promotion directly from the main form view but not the batch form view."
In dues batches, clicking the Promotions button just gives an error message that says "The given key was not present in the dictionary." Promotions and Discounts are working fine in Daily Sales.
This issue has been resolved in version 5.12. All sites have been updated in February 2019. If you experience this error, please chat with support to provide the exact steps that were taken.
Steps to Duplicate
Go to the Memberships tab
Click Enter Membership dues
Click the drop down next to your batch and click Edit > Edit Batch
Enter in Membership information and select the Dues tab