For the command-line logs.
You could check the cli.log in %My Documents%\My Tableau Prep Repository\Command Line Repository\Logs
1. Delete all the files in the folder.
2. Re-run the command line. (Re-produce the error condition.)
3. Check the log.
(And it might not be useful. )
And according to your message,
the error should be here. (need to open the prep flow)