In Chimpegration Classic, certain functions require mapping a field in Mailchimp to the Raiser's Edge Constituent ID. When that is the case and the field is not mapped, you will see an exception as follows, "The constituent ID must be mapped to a merge variable in order to import."
If you are new to Chimpegration Classic, you may need to create this merge field in your Mailchimp account so that you can map your RE field to that location. See the complete list of field recommendations in our Getting Started in Chimpegration Classic section of the User Guide.
If you already have set up your merge field, ensure that it is mapped in your Chimpegration setup.