めったに〜ない
rarely, hardly ever
めったに + negative
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めったに with a negative means "rarely, hardly ever": 父はめったに怒らない, "my father rarely gets angry".
It always needs a negative verb or ない. Without it, the sentence doesn't work: not めったに怒る.
On the scale of frequency: いつも (always) → よく (often) → 時々 (sometimes) → たまに (occasionally) → めったに〜ない (hardly ever) → 全然〜ない (never). たまに is positive and occasional; めったに is negative and rare.
こんなチャンスはめったにない, "chances like this don't come along often", is a common set phrase. In grammar questions, a めったに at the start is a strong clue that the verb must end in ない.
Examples
父はめったに怒らない。
My father rarely gets angry.
この辺りでは雪はめったに降らない。
It rarely snows around here.
最近はめったに映画を見ない。
These days I hardly ever watch films.
彼女はめったに休まない。
She hardly ever takes time off.
こんなチャンスはめったにない。
Chances like this don't come along often.