Even though the Windows 10 May 2020 Update was made generally available in late May, there were still a whole bunch of known issues that caused many PCs to be blocked from installing it. One of those issues was for PCs that have more than one Always On, Always Connected network adapter, such as Microsoft's own Surface Laptop 3 and Surface Pro 7.
The issue was actually fixed with June's round of Patch Tuesday updates, but as usual, Microsoft waited a bit before removing the block. Today, users affected by that issue can now upgrade to Windows 10 version 2004, as spotted by Mary Jo Foley on the Windows 10 release information page.
The bad news is that there's really no easy way to know if you're affected, unless you have a Surface Laptop 3 or Surface Pro 7, in which case Microsoft actually called out your PC. If you're using some other PC that has this type of network adapter, you've just been told that the Windows 10 May 2020 Update isn't ready for your PC. Microsoft doesn't bother to tell you why.
At some point, you'll see in Windows Update that the feature update is available to you, and then, you'll have to opt into installing it. If it still says that your PC isn't ready, hopefully you'll have better luck with the next known issue that gets fixed, hoping that that's the one that was affecting your machine.