You can’t underestimate the importance of …

[ImNotThereAmI]

I’ve recently seen 3 people on tv say this, and also my boss, so now I’m starting to doubt myself. But I’m pretty sure that when you want to say how really very important something is, the saying should be, you can’t overestimate. Eg you really can’t overestimate how much difference this charity money will make to peoples lives. Surely underestimate means its
not important at all? Im not wrong am I? But if I’m right, then why are so many people getting this the wrong way round?

You're interpreting can't as "not able to / impossible to". They're interpreting can't as "shouldn't / must not". I'd say the same as you, but I think either interpretation makes sense.

Yes, you’re right.

It’s similar to “I could care less” which Americans say instead of “I couldn’t care less”. Means exactly the opposite of what they mean.

Oh I hadn’t thought of this!

The person listening to it shouldn't underestimate the importance, because the person telling them about it is saying it's so important, don't imagine you've currently assumed the correct level of importance - it should be more important to you!

The person saying it can't overestimate it enough - although I think this is a case of the wrong word - they would normally say "I can't overstate the importance" i.e. however much I bang on about it, it's still more important than that!

And also what @FlameTulip said

You’re taking it as “you can’t overstate the importance”. They mean “you shouldn’t miss the importance”. It’s unclear but not incorrect

I know the saying as “don’t underestimate the importance of” which does make sense