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