DVS Signal will not go into Serato after Mac OS 10.14 (Mojave) and later if Microphone access is not granted in the Security & Privacy section of your System Preferences.
You can find that setting in the following location:
System Preferences → Security & Privacy → Privacy → Microphone → Serato DJ Pro
Once there, check the box next to Serato DJ Pro to enable Microphone Access.
Note: After granting Microphone access, you will need to close and reopen Serato to restore DVS functionality