So I'm going to go over my read of what happened at Yavin IV, why Luke was allowed to fly, and what was really happening in the airspace around the Deathstar.

A lot of it is informed and extrapolating from the Luke Targeting Computer video on YouTube.

I also want to juxtapose Like with Rey. When people claim Rey is a Mary Sue though I'm not 100% sure what that label intimates I believe whatever skill or power she has is unexplained or impossible to possess based on her in movie characterization, her usage of the force had nuance she hadn't earned etc.

So Luke, lots of experience flying Incom T-16 skyhoppers in high speed canyon runs bullseyeing womprats. At the end of Episode IV, he's tossed into an Incom T-65 X-Wing Space Superiority Fighter on a nod and a wink from Biggs.

So after flying through the magnetic shielding/interference of the Death Star we see the squadrons splitting up for strafing runs and organizing for specific attempts on the mission objective, the exhaust port.

Luke says "I'm going in" and starts a strafing run with Wedge saying pull up Luke and Luke nearly plowing into Death Star before barely making it back into space.

What was this? What happened here?

Remember, Luke's foundational experience was on a skyhopper on Tatooine, and though I am sure he was a star in that scenario, and the flight systems have many similarities, there are some key differences in the space around the Death Star.

Atmosphere

The Death Star was said to have had some gravitational attraction of gases but nothing like a persistent homogenous gas mix a planet might have. So for Luke, any drag that would normally apply as his velocity increased no longer exists. This is ultimately secondary to the real problem,

Lift

In an earth-type atmosphere, when planes enter dives aiming for a point in the ground the have to adjust and force their controls down or else they overshoot their target.

As you're diving, your velocity is increasing and the lift force generated by your wings correspondingly increases. As you pull up out of the dive there's a sort of slingshot effect perceived as you're pulling out of the dive due to this additional lift.

None of this applied when Luke dove down for his attack run on the Death Star. While he was conceivably proficient in operating the controls of an Xwing, Luke had 0 "feel" for what was and was not possible with the fighter and was applying his "feel" for the T-16 Skyhopper nearly resulting in his death.

On a sidenote, the dive = lift was attributed as a major reason the inexperienced kamikaze pilots consistently missed their targets by overshooting them. Even though as the dive started their noses were pointed at the target, through the dive they couldn't muscle the controls to the point they could entirely counteract the lift force and stay on target.

(I have no sources for any of this so if I'm wrong please correct me)

Ok, so next time the camera is on Luke he gets his Kenobi message and makes a force-assisted strafing run, which is much more effective and doesn't nearly kill him.

After that we get the "Enemy fighters coming your way." and Luke's panicked "My scopes negative. I don't see anything."

What was this? What happened here?

He didn't flip the switch. He never turned it on.

I'm postulating... we go back to the T-116 skyhopper, which has a real life analog to a crop-dusting style of plane.

Crop dusters don't have radar. Cessnas don't have radar. Luke has flown a T16 thousands of times. This is his first time in a T-65. He's never turned on, adjusted, dialed in, or worked in any way with an active radar system.

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Even in whatever canon or non-canon simulations were run at Yavin, it's entirely possible that a pilot watching Luke startup the T-65 X-wing, having done it himself 100's of times, sees the kid initiating the startup sequence like a seasoned pro, and they'd never connect the dots.

The other obvious answer is interference/jamming but in the open comms of Red Squadron, no one else repeats Luke's statement of their scopes being negative.

A third option, which just occurred to me, is that he did turn it on, but its radius of investigation was set at 5m or something, a useless number in the Death Star fight while extremely useful in the close confines of a crowded flight hangar. Luke just didnt know he needed to turn a nob to push his scanning out to an effective range.