If a package isn't visible, or can't be added via name, it may be pre-release.
You can find out how to enable these packages with the current version of Unity here.
Before 2020 this was in the Advanced dropdown at the top of UPM, but it has now moved to Project Settings (). You can reach this menu using the cog at the top of UPM.
Pre-release was previously called preview.
As of 2020 the visibility of some packages in UPM has been removed. This is because they are deemed far from being verified, or are only used as a dependency to other packages. You can find more about this here.
These packages will have to be manually added.