For instructions on how to link an account to the Marketing Cloud, click here. If you receive one of the following errors when linking the account, contact support. 

• “ERROR Your account cannot be linked because it is not assigned to a company”.
• “ERROR Your account could not be linked: Your Adobe ID user is already linked to this company with the legacy user.”
• “ERROR An unexpected error occurred while linking your account.”
• “ERROR Your account could not be linked: Your company is not yet enabled for the Marketing Cloud. You may request access here. Error: Organization missing."

If the company has already been provisioned for the Marketing Cloud, the error is a provisioning issue. For resolution to any of errors listed above, contact support. 

If the login company has not been provisioned for the Marketing Cloud, follow the instructions here.