Why I cannot play m4a (AAC) music files on the watch?

I transferred some music via Garmin Express on my watch. The process ended successful. However, trying to find the music on the watch ended up that I get the message that I have to transfer some music-no files found. According to https://support.garmin.com/fr-FR/?faq=kAQRzhwL754A1POp3f9dG7 the watch is supporting m4a. I use the codec AAC but it is not working. Any ideas?