By default, Spotify does not support lossless files, including ALAC in .m4a extension.
It does play AAC files that are also in .m4a extension.
However, according to this it should be possible to play ALAC files too, if you're on Windows 10 (it supports ALAC codec). I am not sure if it will work for everyone or not, but you can check it out. 🙂
Hope this helps, have a nice day!
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.