Causes of the “File Open Error” are:

1. There are not enough permissions on the files. All files need full access permissions. (rwxrwxrwx)
2. The file is open. Make sure files are closed in all other applications before merging.
3. Wrong filename or directory path. (Most common and easiest mistake to make.)
4. File is checked as Read Only (Windows). Go into your General Properties and uncheck Read Only.