に足らない / に足りない【にたらない / にたりない】

に足らない means "not worth": the thing does not deserve the effort of worrying, fearing, believing or discussing it. It goes after the dictionary form of a verb, or after a する-noun such as しんらい信頼. にたりない足りないis the same expression; に足らない is the more common and more literary of the two.

The tone is dismissive: the thing falls below the line where it deserves attention.

Set phrases

とる取るにたらない足らない means "trivial", "of no importance". It is the use you meet most, usually before a noun. おどろく驚くにたらない足らない is "not surprising": you expected it.

The affirmative, にたる足る, means "worth" or "enough to".

Register

に足らない belongs to essays, commentary and formal speech. In conversation, たいしたことない大したことない or きにしなくて気にしなくていい say the same thing.