macOS 10.15 Catalina & macOS 11 Big Sur requires the user to allow Serato DJ Pro Or Lite permission to access removable volumes. If you don't allow this access you may not be able to access music files that are stored on external drives.
You should be prompted by macOS to allow Serato DJ Pro or Lite to access removable volumes when you launch the software with an external drive connected, or if you connect one while the software is running. Click 'OK' to allow:
Without doing so you may be unable to access files from external drives. If you chose 'Don't Allow' or ignored the prompt, you can also retrospectively adjust your settings to allow access (you will need to 'click the lock to make changes' and enter your Mac user account's log-in details to adjust the settings below):
System Preferences → Security & Privacy → Privacy → Files and Folders → Serato DJ Pro (or DJ Lite) → (Removable Volumes)