ORA-19504: failed to create file ORA-unable to lock file - already in use

Two/Three Possible causes :

1 .some grants/permission issue on the folder. First check all grant/permission is OK.

2. File already exists. Check if file exists

 2.a or 3. File may exist, because of non-unique file name. In this case, you should specify a tag similiar to "_%U" to make the filename unique:

  backup database format '%PATH%\xxx_%U.bak'