What is Sidecar?
Sidecar is a brand new remote display method that makes iPad the second Mac display and mirrors Mac to iPad to have the two screens displaying the same content. Without any app needed to load either on the Mac or the iPad, you can do operations on your iPad that you usually do on your Mac. To use Sidecar between Mac and iPad, you need two basic things: an iPad running iPadOS and a Mac running macOS 10.15 Catalina.
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How to Solve macOS Catalina and iPadOS Sidecar not Working?
As Sidecar feature is quite new, Sidecar not working/not showing up error may happen to some of you. So, here we gather all possible solutions to this kind of problem for your reference.
1. Sidecar won't work because the distance is too long
You can enable Sidecar mirroring between macOS Catalina and iPadOS in a wired or wireless way. If your supported iPad is within 10 meters of your supported Mac, you can connect the two devices with a Lightning or USB-cable, or through Bluetooth. If the distance is longer, it is impossible to make the two connect to each other, and thus Sidecar not mirroring error happens.
2. Your Mac won't mirror to iPad because you didn't log into the same iCloud account
Also a basic requirement, your Mac Catalina and iPadOS need to be logged into the same iCloud account to make the Sidecar work between them.
Log in with the same Apple ID > enable two-factor authentication on both iPad and Mac > activate the Bluetooth on both devices and make sure they are under the same WiFi network.
If you connect your iPad to Mac with cable, just wait for your Mac to recognize your iPad and then Trust the device and enter a PIN if asked.
3. Sidecar not working or not showing up because of the incompatible Mac and iPad devices
Sidecar feature on macOS Catalina is limited to many of Apple's new Macs, only some of them support Sidecar. At present, Sidecar is available to iMac 27 (Late 2015) or newer, MacBook Pro (2016) or newer, Mac Mini (2018), Mac Pro (2019), MacBook Air (2018) or newer, MacBook (Early 2016) or newer, iMac Pro (2017) or newer.
As for iPad models, we are not clear if all the iPad devices will work with macOS Catalina Sidecar. Luckily, we know that iPadOS is the basic thing to Sidecar mirroring, and iPadOS will surely run on types of iPad models. However, Apple has decided to restrict Sidecar only to iPads with Apple Pencil support according to the latest news. That is to say, only iPads shipped with iPadOS 13 and with Apple Pencil support will allow for Sidecar mirroring from Mac to iPad. The devices include iPad (6th generation), iPad (7th generation), iPad mini 5, iPad Air (3rd generation) and iPad Pro (all generations).
Therefore, if your devices are not included in the list mentioned above, probably the Sidecar won't work between macOS Catalina and iPadOS or even Sidecar option won't show up.
4. If your iPad doesn't show up in the AirPlay list on Mac, Sidecar not working error happens
Commonly, by clicking on the AirPlay icon in the upper right corner of your Mac's Menu bar, you can select your iPad to display your Mac contents. However, even when your iPad is hardwired to your Mac, there may be no iPad option in the AirPlay list for Sidecar mirroring. You can try to manually fix the Sidecar not showing up problem by following the method:
1. Open Settings on your Mac.
2. Click on Sidebar.
3. Click on Devices under Connect to.
4. Select your iPad from the dropdown menu.
Worth mentioning, you are highly suggested to plug your iPad into your Mac as Sidecar not working error often occurs when connecting iPad and Mac via Bluetooth.
5. Sidecar is not working for your apps
From Apple official site, there are a few apps that support Sidecar mirroring from macOS Catalina to iPadOS as of now. The Sidecar supported apps include Adobe: After Effects, Illustrator, Premiere Pro; Affinity Designer & Affinity Photo; Cinema 4D; CorelDRAW; DaVinci Resolve; Final Cut Pro & Motion; Maya; Painter; Principle; Sketch; Substance Designer & Painter; and ZBrush. So, if you can't use Mac apps on an iPad, the possible reason would be the incompatible app.
To sum up, Sidecar mirroring is a unidirectional way, namely, you can only Sidecar mirroring Mac to iPad rather than mirroring iPad to Mac, because Sidecar is controlled by a new feature on macOS Catalina rather than the iPad using iPadOS. If you want to screen mirroring iPadOS to Mac, you can try 5KPlayer.