Key Sync doesn't work.

The key of the track might not have been analyzed.
To use Key Sync, this must be done in advance. Follow the instructions below to analyze the key using your DJ application.

■ For rekordbox
1. Right-click the track you want to analyze.
2. From the menu, click [Analyze Track].
3. Check [KEY] on the displayed window.
4. Click [OK].

For details, see the rekordbox Operating Instructions, available [here] .
■ For Serato DJ Pro
1. Start Serato DJ Pro with your controller or mixer disconnected.
2. Click the gear icon on the left side in the middle of the screen.
3. Check [Set Key] and close the screen.
4. Drag and drop the track you want to analyze to [Analyze Files].

For details, see the Serato DJ Pro Manual, available from the Serato website [here].