RMAN on Windows requires UNC paths for network shares rather than mapped drive letters, eg:

RMAN> duplicate database to TESTDB backup location ='\\SOURCESERVER\Y\BACKUPS\MASTERIMAGE\20170630';

Refer to MOS Docs:

Backup failing on mapped drive on windows (Doc ID 2160858.1)
How to Configure RMAN to Write to Shared Drives on Windows NT/2000/2003 (Doc ID 145843.1)