Please make sure that you have permission to write and modify files at
C:\Program Files\Mp3tag\
To take ownership and have the correct permissions set, right-click on this directory in Windows Explorer and use 'Properties > Security' to modify the permissions.
What does "now" mean exactly? What are you trying to do?
There is not need to write or modify files in the standard program folder. Because these folders are protected by windows you get this error.
How did you install MP3Tag? Standard installation or portable?
In Mp3Tag click on File -> Open Configuration folder and tell us which folderpath you see then.