Hi Hel
Backing up is a separate issue. If you have any version of AccountEdge on Mojave you should be able to backup. There are some things that prevent backup however these are usually based on settings on your computer rather than the application AccountEdge. For example if you don't have write permission or you use a long path or a long file name.
Try to backup your file as test.zip to your desktop and see if that works. Desktop does not have permission issues and a short path and short file name should ensure this works. If it does, then experiment with other locations. File name should not exceed 32 characters and path and file name should not exceed 255 characters.
Regards
Gavin