Data 360: Error "Unable to complete setup due to internal error" when entering Marketing Cloud Engagement credentials in Data 360 Setup

See the following reasons that have been observed as a root cause for the error "Unable to complete setup due to internal error".  Review and implement these points to resolve this issue.

User is missing Administrator and Marketing Cloud Administrator roles

Set Up Marketing Cloud Connection in Data 360 outlines the steps to following to connect Data 360 to your Marketing Cloud Engagement account. As per this Help documentation, the Administrator and Marketing Cloud Administrator roles are required. If the user whose credential you have entered in Data 360 Setup is missing either of these roles, you will see the UI error  "Unable to complete setup due to internal error". Please ensure both roles are assigned to your Marketing Cloud Engagement integration user.

Note: both these roles are generated by default when your Marketing Cloud Engagement account is provisioned. If you have edited either of these roles to remove permissions that should be allowed by default, this can also result in an error returning. In this scenario,  you would need to allow the default permissions that you removed from the roles.

Enterprise-level business unit is not set as default business unit

The integration user in Marketing Cloud Engagement must have the enterprise-level business unit set as default.
See Manage Business Unit Assignments for Marketing Cloud Users to learn how to confirm or update a user's default business unit.

Re-activating Sandbox Connection from Production missing Business Units

Re-activating the connection checks that you have access to any Data Streams that are replicated from Production. If you are missing Business Units you will run into the Internal Error message. You can delete the Data Streams, if you do not want to add the Business Units to the Sandbox Connection.