The Application object needs to be a generalization of 'FileDocument'. I think that is the problem here. Open the object and select generalisation to filedocument.
If you don't need different languages in your document, then it's not needed to specify the language for the user that is invoking the microflow. You could also use system default in the doc settings.
Take a look at this how to: https://docs.mendix.com/refguide7/how-to-create-your-own-documents
That’ll help you understanding the filedocument enitity and how to use it.
Thank you both for your clarification and explanation. Once I associated the generalized the application to “FileDocument”, the MicroFlow worked as intended. Thanks again!