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Without taking risks, you get nothing.
Literal
Adventure [object-を] don't-do-if, nothing not-obtain.
冒険をする ('to take a chance, to have an adventure') uses 冒険 ('adventure, risk-taking') as a noun. The negative conditional ~なければ ('if not') expresses 'unless.' 得る ('to obtain') in the potential negative 得られない = 'cannot obtain.' A classic 'no risk no reward' saying.