Referencing a level 6 Bladesong Wizard, engaged in combat.
The Wizard would is attempting to leverage the level 6 Extra attack feature.
"You can attack twice, instead of once, whenever you take the Attack action on your turn. Moreover, you can cast one of your cantrips in place of one of those attacks."

The question revolves around the usage of the attack action.
If the player states he is taking the attack action, he can swap out either attack for a Cantrip of his choosing, that part is clear and understood.

The primary example to reference is, the wizard is 20 feet from his target, and wants to take the attack action.
With this attack action, he wants to replace the first 'attack' and cast Firebolt, then move closer and make a melee attack.

My understanding is, in order for that to occur, the player would be need to be making one of the following 'attack actions': a weapon attack with a melee weapon, ranged weapon or improvised weapon.

If the player is not making one of those attack actions, the player would have to cast Firebolt as a regular action (spell action) and would not trigger the Extra attack feature.

Is this an accurate assessment, or are we off on this?

I also see RAW exactly what your assessment was.

Quote from yaWgnorW >>

The primary example to reference is, the wizard is 20 feet from his target, and wants to take the attack action.
With this attack action, he wants to replace the first 'attack' and cast Firebolt, then move closer and make a melee attack. 

This is fine.

When a level 6+ Bladesinger takes the attack action they have two attacks.  For other characters these attacks have to be weapon attacks or unarmed strikes, but the Bladesinger is allowed to replace either attack with a cantrip.

When you do this you have still taken the attack action, and can break up your movement the same as with two weapon attacks.

Quote from Gruntler >>

Quote from yaWgnorW >>

The primary example to reference is, the wizard is 20 feet from his target, and wants to take the attack action.
With this attack action, he wants to replace the first 'attack' and cast Firebolt, then move closer and make a melee attack. 

This is fine.

When a level 6+ Bladesinger takes the attack action they have two attacks.  For other characters these attacks have to be weapon attacks or unarmed strikes, but the Bladesinger is allowed to replace either attack with a cantrip.

When you do this you have still taken the attack action, and can break up your movement the same as with two weapon attacks.

Thank you for the response. My challenge is in understanding the 'pre-requisites' for the attack action, if there is any. The ability states that you get a second attack when you take the attack action. This initial attack action is what permits the player to use the Bladesinging Extra attack features.

I guess the ultimate question is, how can the wizard make an attack action without actually being able to make an attack? 

Trying to look from a different view, is this basically stating that a Bladesinger can just effectively say 'I use my attack action to cast a cantrip (normally this is a spell action), and for my extra attack use my movement and make a melee attack'? - If its this simple I can understand it.

You don't actually need to be able to attack; what your player is doing is choosing to take the Attack action on their turn, this then allows them to make (up to) two attacks, one of which can be a cantrip in any order.

Whether or not they can actually complete all of the attacks is irrelevant; they don't have to use both attacks, or they could find one of them is out of range so it automatically fails (can't hit something that's not in range).

Now of course there's no reason to declare the attack action if you're only going to be able to cast the cantrip, as you can just use your action to do that on its own, but there's technically nothing wrong with doing it within the attack action.

It's worth keeping in mind that the attack action and actually rolling attacks are not the same thing; the former lets you do the latter, but you can also roll attacks as bonus actions, as part of spells, reactions etc. It's just a confusing use of the same word for both things.

It is that simple yes.  You're not replacing the Attack Action, just one of the two attacks that it gives you.

Similar to how a Fighter with two attacks can use the first to make ranged weapon attack, and then run in to make a melee weapon attack.  Except instead of a ranged weapon attack you cast a Cantrip instead.

Quote from Haravikk >>

You don't actually need to be able to attack; what your player is doing is choosing to take the Attack action on their turn, this then allows them to make (up to) two attacks, one of which can be a cantrip in any order.

Whether or not they can actually complete all of the attacks is irrelevant; they don't have to use both attacks, or they could find one of them is out of range so it automatically fails (can't hit something that's not in range).

Now of course there's no reason to declare the attack action if you're only going to be able to cast the cantrip, as you can just use your action to do that on its own, but there's technically nothing wrong with doing it within the attack action.

It's worth keeping in mind that the attack action and actually rolling attacks are not the same thing; the former lets you do the latter, but you can also roll attacks as bonus actions, as part of spells, reactions etc. It's just a confusing use of the same word for both things.

Quote from Gruntler >>

It is that simple yes.  You're not replacing the Attack Action, just one of the two attacks that it gives you.

Similar to how a Fighter with two attacks can use the first to make ranged weapon attack, and then run in to make a melee weapon attack.  Except instead of a ranged weapon attack you cast a Cantrip instead.

Just wanted to say thank you to each of you. I think I'm good at this point with your explanations and helping me understand. Appreciate it!