This can be caused by many issues.

Some of the answers are correct, but in in all the answers I never come cross of another obscure root cause and that is at the "Object Ownership" turning on "ACLs enabled".

This can happen, that you are working on shared AWS Account and that option to be turned on by less knowledgeable colleague.

And the data copied there was with a very different identity. And your role is not the "owner" and thus has zero access to object (it can see it, but can not download it).

In this case go:

Bucket > Permissions > Object Ownership > Press "Edit"

And select: "ACLs disabled (recommended)" instead of "ACLs enabled".

Like this:

Either you have a hunch to check. Or you upload something and if you can download it (you being the owner), but you can not download other object then this could be (or it can be folder, prefix too policies, but you can check that too).