It’s a little odd, but you have to import the audio in a very specific way otherwise you get that error.

1. Create a new project
2. In the “Tracks” tab select “Audio Recorder”
3. In the top left, click the icon that kinda looks like a brick wall
4. In the top right, click the icon that looks like a loop
5. Find your audio file and then click and hold it
6. Drag the file onto the only audio track in the project
7. Click the down arrow at the top far left and then “My Songs” to save and exit
8. Tap and hold on the song you created and then tap “Share”

You should be able to export as a ringtone then.

Someone also posted that you can directly export a song less than 30 seconds from the files app in iOS 26, completely removing the need for GarageBand.