merging audio files

so frustrating, l have followed the instructions online and have successfully merged the m4a files into one track, however when I try to export it as mp3 , m4a. it doesnt allow, saying l have to download LAME. when l click download it takes me to website as attached, and says in MAC it is already built into Audacity :angry:
l can export as an aiff file but that takes up many times the space, where an mp3 or m4a would be sufficient, please help me get this audio mergering done, btw tried quicktime and trying to drop a 2nd m4a file onto the 1st m4a quicktime window did not work, although mp4s merging was no problem.
when trying to save as m4a.png
when trying to download LAME, for mp3 export.png
Screen Shot 2019-06-24 at 00.21.46.png
Screen Shot 2019-06-24 at 00.13.05.png

Have you got the latest version of Audacity which is 2.3.2 ? That us the first version in which we built in LAME.

BTW LAME is only for MP3 export - for MP4/AAC yoy need the optional FFmpeg library.

But a workaround for you would be to Export as AIFF or WACV and use iTunes to covert to MP3 or MP4.