Context determines whether "could not" is replaceable with "can not". The modal verb could is preferred to formulate more formal and polite requests, the meaning in both versions is the same.
- Could I have a glass of water?
- Can I have a glass of water?
In English when we talk about ABILITY in the PRESENT, we use can, can not, or cannot. The construction be able to would also be possible here.
Machines cannot predict a person's IQ based on their physical appearance.
Machines are not able to predict a person's IQ based on their physical appearance.
In the OP's example, the situation appears to refer to an ABILITY in the PAST (without the actual source there is no way to be certain). The author seems to claim that in the past it was difficult to know with reliability whether someone who scored high on the trait of generosity would behave generously in a specific situation.
This means that one could not predict with confidence.
If could not is replaced with can not, the implication is the situation has since changed. It is now possible to predict with greater confidence using better tests.
This means that one can not predict with confidence whether a person who scored high on the trait of generosity would behave in a generous manner in a given situation.