Chances are that your trial expired. Try exporting as an AVI, instead of mp4.
If that fails, install this version, and try both formats. Some features will not be available unless a license is purchased.
Remember to Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar.