I wonder whether your best option is to register at and repost your question to the dedicated Samplitude forum here? If you do so, I would strongly suggest giving full details of your computer, soundcard/audio interface etc., etc. to enable someone to assist you.
I think that this happens if you another program open that is using your audio card with a different sample rate, like Music Maker or a video program. Make sure to close them and try again or try after rebooting.