Trados Studio 2015, by default, will not process InDesign’s IDML files created with versions CC 2017, 2018 and 2019. Here is the screen you will get when you drag the files for translation:
The two Warning messages read “Pre-Scanning failed to identify the file type, the following messages were generated:” and “Pre-Scanning Error: The selected IDML file was created by an unsupported version of Adobe InDesign and will not be processed”.
The Work Around
Before you add the files, click on File Types…
Now, search for Adobe InDesign CS4-CC IDML, expand it, and click on Common. On the right panel, you will click on Process unsupported file versions.
Now, you should be able to process files without any known issues, for at least version the versions mentioned above.