〜ざるを得ない
have no choice but to, can't help but
Verb ない-form minus ない + ざるを得ない (する → せざる)
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ざるを得ない means you're forced to do something, usually against your wishes: 雨なので、試合を中止せざるを得ない, "it's raining, so we have no choice but to cancel the match".
ざる is an old negative, so it's literally "can't get not doing". Build it from the ない-form: 従わない → 従わざる, 認めない → 認めざる. The exception is する, which becomes せざる: 中止せざるを得ない.
It's more formal and more reluctant than しかない. The situation leaves no choice: an order, the weather, evidence, circumstances.
With 認める and 言う, it's a common way to concede a point: 認めざるを得ない, "I have to admit it".
Examples
雨なので、試合を中止せざるを得ない。
It's raining, so we have no choice but to cancel the match.
上司の命令なので、従わざるを得ない。
It's my boss's order, so I have no choice but to obey.
体調が悪いので、今日は休まざるを得ない。
I'm not well, so I have no choice but to take today off.
証拠があるから、認めざるを得ない。
There's evidence, so I have to admit it.
終電がないので、タクシーで帰らざるを得なかった。
The last train had gone, so I had no choice but to take a taxi home.