In my test environment, the purge command did not give any output.
When using the commandline tools from the GUI, you have to specify the passphrase (--passphrase="mypassword"), because there is no way to provide it when the operation is running.
Duplicati stores the complete path for each file and folder and it expects a complete path as input for operations like purge. Wildcards are allowed.
Replace unwanted_file.txt with "*/unwanted_file.txt" (including double quotes).