Hi, welcome to the Magix forums.

First, please read this post and give us all the information requested there.

Second, you cannot save a Photostory project as an mp4 file but you can export a project to create an mp4. Once you have created your project on the timeline, choose the Export button (top right hand corner) and the screen you then see should look like this:

That "Format" option has a drop-down with various choices; select MPEG-4 and then make the Quality choice.

HTH

Jeff