A creature with a Bardic Inspiration die can roll it to add the result to an ability check, saving throw, or attack roll, while the Divination wizard's Portent feature overrides the roll before the roll is made.

Because the roll is never been made, does this mean that a roll replaced by Portent cannot be improved by the Bardic Inspiration die?

Yes, in fact you can.

According to official 5e rules designer Jeremy Crawford, in this tweet:

@IgnatiusJRiell The portent die is intended to replace a d20 roll only, not any modifiers applied to it.

Portent only replaces the die roll, not the entire check; bonuses can then be added, including Bardic Inspiration.

Bardic Inspiration is used to boost the activity a PC is carrying out now:

Once within the next 10 minutes, the creature can roll the die and add the number rolled to one ability check, attack roll, or saving throw it makes.

The Divination wizard's Portent feature (PHB, p. 116) is used to replace a roll made, with the value they originally rolled:

You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls. You must choose to do so before the roll, and you can replace a roll in this way only once per turn.

Since Portent is not a saving throw, attack roll, or ability check, and there is no roll when Portent is applied, they cannot be affected by Bardic Inspiration.

However, this can be allowed by the DM if they so choose.