Can couldn't do and couldn't have done mean the same?

No, they are not the same. The former means having no ability to do it. The latter means having no possibility / opportunity to do it.
I couldn't afford a new car when I first got out of school. So I bought a used car.
He couldn't have done it. He was in London yesterday.

No. They're not the same. Past failure is just "couldn't", not "couldn't have".

I tried ..., but I couldn't do so.

"couldn't have" is for imagined past inabilities so there is an if-clause, explicit or implicit.

A: I just finished reading all of this 850-page book!
B: Wow! (If I had tried that) I couldn't have done that!

I bought a/an used car?

shravanmmI bought a/an used car?

"a". It goes by sound, not by spelling.

"used" is pronounced "yoozd" with the initial consonant "y", so use "a".

CJ

what do you think of these examples

'I looked for this book everywhere, but I couldn't find it.'
'I looked for this book everywhere, but I couldn't have found it. It was impossible for me to find this book, because my brother had taken it with him.'

TommyekWhat do you think of these examples?

Tommyek'I looked for this book everywhere, but I couldn't find it.'

Perfect.

Tommyek'I looked for this book everywhere, but I couldn't have found it. It was impossible for me to find this book,

in order not to make a mess here, here is proper version

'I looked for this book everywhere, but I couldn't find it.' It was impossible for me to find it, because my ability to find things is tor poor.
'I looked for this book everywhere, but I couldn't have found it. It was impossible for me to find this book, because my brother had taken it with him.'

TommyekI looked for this book everywhere, but I couldn't find it.

Fine.

TommyekI looked for this book everywhere, but I couldn't have found it

Not good. "couldn't have found" is not related to looking for it. "couldn't have found" is related to "my brother had taken it". You've put the wrong facts together in the same sen

Okay, I think I've got it. Do these examples illustrate the point?

I couldn't translate this text. It was far too complicated.
I couldn't have translated this text. It was as easy as ABC. However, my computer, where I did my translations, had broken down.

TommyekOkay, I think I've got it. Do these examples illustrate the point?I couldn't translate this text. It was far too complicated.I couldn't have translated this text. (It was as easy as ABC. However,) my computer, where I did my translations, had broken down.

Yes, these illustrate the point. Note, however, that there is no need for the material in parenthe