I’m trying to upload a custom sound, but for some reason TikTok won’t let me?

So I’m trying to upload a slideshow to TikTok, and I want to use a little music clip I made for it, which is obviously not already on TikTok.

I tried the “upload a video with your sound as private, then you can use the sound on other videos” trick I’ve seen people mention online, but for some reason whenever I try to press on the spinning disk at the bottom right is the screen, I just get an error pop-up saying “Sound Not Available”.

Does anyone know why this is? How can I fix it?