A colloquial expression meaning 'not entirely absent' or 'it's not that ...'; used to give a qualified or mild agreement (e.g., 'it's not impossible' / 'there's some truth to that').
彼の意見、なくはないけど、もう少し根拠が必要だ。
His opinion isn't without merit, but it needs a bit more supporting evidence.
その方法が成功する可能性はなくはないが、リスクが大きい。
It's not impossible that that method will succeed, but the risks are high.
高くて買えないわけではなく、なくはないけど予算が足りない。
It's not that I can't afford it; it's not impossible, but the budget is lacking.
条件が揃えば成功しなくはないと思う。
I think it's not impossible to succeed if the conditions are right.