Assuming you mean the 'Spectrum visualizer' in MMM 2023 it is a tool, not a visual effect which can be added to a music video.
To create a music video you add a video clip or images to track 1 - see image below - the video clip must be a supported format, ie AVI, WMV or MXV the custom Magix video format. Images in JPG, PNG and BMP are supported
You have to treat MMM as a video editor in that you have to put the visuals on the arranger just like the audio. I suggest track 1. Drag the visual that you have selected onto track 1 or another empty track. Make sure to get visuals right up to the end of the audio.
Finally, you will have to export as a video, not audio, because you are now making a movie.