Additional InformationFor more information about this topic, see How to Migrate Tableau Server from Local Authentication to Active Directory by Using Site Migration in the Tableau Community. The document contains full details of the error messages and how to fix them:
"The entire import will fail if even a single ... mapping is invalid. For systems with hundreds or thousands of user mappings that are difficult to accurately verify manually, it is a best practice to leverage Tableau Desktop to machine-verify the accuracy of your mapping file. (For a large site that takes many hours to import, this step can save you hours of wasted time if an import were to fail at the end.) When you think your mappingsSystemUserNameMapper.csv is ready, try connecting to it with Tableau Desktop’s CSV data source method, and join it to your target system’s PostgreSQL repository, specifically the system_users table, which will contain entries for each mapped Active Directory user. Establish a left-outer join between mappingsSystemUserNameMapper.csv and system_users and use this to create a new sheet which then verifies that each incoming Local user’s Target Name column truly has a valid, matching Active Direcotry account Name in system_users."