Sdkmanager can't install Jetpack4.2.1

Now I want to install JetPack 4.2.1 in Jetson Nano(with emmc, P3448-0020), but I can not find JetPack4.2.1 in TARGET OPERATION SYSTEM in SDK Manager. What’s the reason for this?

You can download older versions in the archive:

What I am wondering if you could remove the current jetpack and reinstall it with a specific version like here:

Thanks for your reply.
I can find JetPack 4.2.1 in SDK Manager today. Maybe Nvidia fixed the bug yesterday.