Built in display not working from MacBook Pro to MacBook Air

AlexSchweizer wrote:

I tried to do the built in display sharing from my MACBOOK PRO to my MACBOOK AIR to use my MacBook Air as a second monitor, but my MacBook Pro does not recognize the device to do so.

The funny thing is that my MacBook Air does share screen, but my MacBook doesn't.

No Mac notebooks ever made support being used as hardware monitors for other devices. So I assume that you are talking about AirPlay to Mac. This is a fairly recent feature – much more recent than the ability to AirPlay from a Mac to a device such as an Apple TV set-top box.

Continuity features and requirements for Apple devices - Apple Support

Your M4 MacBook Pro meets the requirements to be an AirPlay Receiver. Your MacBook Air can only meet them if it has a hardware model year of 2018 or later, and is running Monterey or later. If it doesn't meet those requirements, that would explain why it can use the M4 MacBook Pro as an AirPlay display, but not vice versa.

There are third-party applications that you can buy to turn older Macs into "AirPlay Receivers". It is my impression that these applications use the older AirPlay v1 protocol and support a maximum resolution of 1920x1080, but I am mentioning them in case they are of interest.

AlexSchweizer wrote:

I do can share screen on my iPad, I can use the iPad as a second screen and it works perfectly, but no my old MacBook Air, it doesn't even recognize it.

I also cannot share from MacBook Air to iPad

Sidecar is the feature that lets you use an iPad as an extra display for a Mac.

Continuity features and requirements for Apple devices - Apple Support

To support it, your MacBook Air would need to be one released in 2018 or later, running Catalina or later. Since you have had no luck using your MacBook Air as an AirPlay Receiver, or using Sidecar with it, that might suggest that it has a hardware model year earlier than 2018.

AlexSchweizer wrote:

• MacBook Pro M4

• MacBook Air dual-core intel core i5

I tried to do the built in display sharing from my MACBOOK PRO to my MACBOOK AIR to use my MacBook Air as a second monitor, but my MacBook Pro does not recognize the device to do so.

I would like to use my MacBook Air as a second monitor, but if It doesn't work, I might just sell it then.

Thank you!

To use one Mac to mirror or extend its screen to another Mac, ensure both Macs are running a compatible operating system and are on the same Wi-Fi network, then

use the Control Center's Screen Mirroring feature to select the target Mac as a display.

Extend or mirror your Mac desktop across multiple displays

Use AirPlay to stream video or mirror the screen of your ...

AlexSchweizer wrote:

I would like to use my MacBook Air as a second monitor, but if It doesn't work, I might just sell it then.

If you decide to sell it, see:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support