「！」。
He shouted, "This is a battle I can't afford to lose!"
Literal
He [topic-は] 'lose-cannot battle [copula-なんだ]!' [quotative-と] shouted.
負けられない戦い ('a battle that cannot be lost') uses the negative potential of 負ける. なんだ is the emphatic explanatory ending (なのだ). と叫んだ ('shouted [saying]') uses the quotative と for direct speech. The phrase 負けられない戦い is famous from Japanese sports broadcasting — it became a catchphrase during World Cup coverage.