。
There's no use making excuses.
Literal
Make-excuses-even useless is.
~ても無駄だ is the fixed 'no use X-ing' pattern — literally 'even if X, it's useless.' 弁解する is a noun-する compound meaning 'make excuses, justify oneself' — slightly more formal than 言い訳する, with a hint of 'plead one's case.'