HELP! Motion 5 error 12143

I just spent a fair amount of time creating a motion project. It will not allow me to save (no explanation given) and when I try to export my project I get an error window stating

Export Failed

The operation could not e completed because the output file could not be created (error -12143)

What does this mean? Am I going to have to loose the project and start from scratch? *insert VERY worried face here*

Thank you soooo much for your help!