Users have checked the following:
- Promotion Code is not yet expired
- Name of the promotion code used is correct
This is happening because there is a duplicate Promotion Code that is set to Inactive. Check the list of Promotion Codes and check the option Show Inactives to check for duplicates.
To verify if there's an inactive promotion code with the same name, perform the following steps:
1. Navigate to Lists > Marketing > Promotion Codes.
2. Check Show Inactives.
3. Click the Name header to sort the list by name.
4. Verify if there's an inactive promotion code that uses the same name.
To resolve the error, rename the inactive Promotion Code.