First, I'm not a representative of Microsoft, just a fellow user of Movie Maker, so I won't be able to do anything in the program to fix your issue.
Most often, when a project will not render as a movie, there are two main causes:
- The source files aren't 100% compatible with Movie Maker. When I have that issue, I convert all the video clips to .wmv format and all the music files to .wma format using Freemake Video Converter. After that, I usually have no problems saving.
- The project is too complex. When I can only get it to get to about 50%, I split the project into two parts and try to save each separately as movies. If that works, I then create a third project and join the two parts together. The easiest way to split a project is to make multiple copies of the .wlmp file. In the first copy, delete clips from the end until it will save as a movie. Call this Part 1. Now open another copy of the project file, and delete the parts that are already saved. See if you can save the second half as a movie. (If for some reason, you can't make it work with half, try it in thirds, fourths, etc.)