Mixxx receives a bad timecode signal (likely too low/loud).
The signal quality widget should show a green donut for a perfect signal
You must set the input mode of your sound card to phono if you have standard turntables. Many modern turntables have a Line/Phono select switch. If yours does, make sure it matches the input or switch setting on your sound card. Also check if your needle needs cleaning.
In the Mixxx online manual you`ll find a wiring diagram for turntableshttp://mixxx.org/manual/latest/chapters/setup.html#turntables