Cannot assert something definitively; unable to say with finality. Used when one is not certain about a fact or outcome.
その証拠だけでは彼が犯人だとは言い切れない。
With just that evidence, you can't definitively say he's the culprit.
今の状況から将来のことを言い切れないのは当然だ。
It's natural that you can't say for certain what will happen in the future given the current situation.
データが不十分なので、成功すると言い切れない。
Because the data are insufficient, we can't say for sure that it will succeed.