Diagnose 125v 20a receptacle voltage
So in the breaker box, you have black, red and ground but in the outlet box you have black, white and ground.

Odds are that you have a junction box somewhere where the red wire is connected to the white wire. Since you capped the red wire in the breaker box, the white wire is not connected, just floating.

I’m also assuming you checked the voltages with a digital voltmeter. In this case, you’re seeing a phantom voltage, induced from other wires. The voltmeter provides very little load.

To test this, plug an outlet tester into the outlet, it should show “open neutral”. You can also measure the voltage with the tester plugged in. You should now see 120 volts (or nearly that) on the white, conducted through the tester.

Assuming all of that proves true, you need to land the red wire onto the neutral bus in the breaker box. I’d also suggest marking the end of the red wire as white, either with paint or tape.