Converting M4A file to MP3 - Selected files could not be found
Another solution: 


Select View | As Songs to display the library as a list of tracks (i.e. not organised by album). This is the key part.
Then hold the Control key and select the songs to be converted by clicking each one.
Lastly, select File | Convert | Create MP3 Version.


Before doing the above, make sure that the sound encoder is set to MP3 under Edit | Preferences | Import Preferences | Import Using ...