Authenticate Marketing Cloud User to Distributed Marketing application

To resolve this issue and re-authenticate Distributed Marketing to the associated Marketing Cloud business unit complete all bullets below:

• Login to the Sales Cloud instance where Distributed Marketing has been installed. Make sure to log out of any other Salesforce Instances. (Do not use Incognito)
• Travel to the 'Distributed Marketing Administration' tab.
• Under the 'Connect Business Units' panel and 'Connection Status' column header, click the 'Complete Login' hyperlink. 
• Once directed to the 'Named Credentials' page, click 'Edit', select the 'Start Authentication Flow on Save' checkbox and then click 'Save'.
• A screen will prompt login to the Marketing Cloud. The credentials used to login through this screen will create an authenticated channel from Distributed Marketing to the Marketing Cloud business unit.
• Head back to the 'Distributed Marketing Administration' and click Update Connection Information

Other troubleshooting steps for first time setup – If these were not performed in the original BU setup, deleting and recreating the BU would need to be done. Not doing the follow steps correctly makes the Named Credential in a broken state.

• Confirm the TSE in the Named Credential matches your Marketing Clouds TSE. If it does not, this indicates the Business Unit connect was made incorrect.
• If you are using a Marketing Cloud API user with a space in their name, you will need to add an underscore as a placeholder for a space. EX: First Last would need to be First_Last
• Confirm you select the correct environment. When creating in a Sandbox select Sandbox, Production will choose production. Not doing so directs the DM App to the incorrect login page.
• Clear cache and cookies.

Additional Reference

• Create Marketing Cloud API User