Can u be terminated if u are unavailable for a load as a driver and the boss knows u are unavailable and didn't inform him that your back available?

Yes.  Tennessee is an employment-at-will state.  You would only have a claim against the employer if there was an unlawful motive for the termination such as unlawful discrimination or unlawful retaliation.  The only possible claim, based on these short facts only, is if you were unavailable because Hours of Service regulations from the DOL.  If that is what happened or there are other pertinent facts, contact an experienced employment attorney as soon as possible.