VocALign needs to find a signal in the first 0.25 seconds of the guide and dub audio. If there is a very low level signal or digital silence this error will be displayed.
You can prevent the error by adjusting the start of the selected audio.
To adjust the start find the gray bar at the start of the signal and click and drag the bar with your mouse.
This note shows the issue with VocALign Project on Pro Tools similar steps apply to VocALign Pro and VocALign Project on other DAWs.