A recent issue that I have noticed is that some installs of Revit 2019 are missing the content libraries. After conducting some research on this issue it appears that this is not always the case, however during some installs the content libraries are being installed to the wrong location. So, if this issue has affected your install, you can check and sort it now.
The content libraries should be located at C:ProgramDataAutodeskRVT 2019Libraries, but for some reason they are instead going to: C:ProgramDataAutodesk.
I fixed this issue on my machine by moving the Libraries folder to C:ProgramDataAutodeskRVT 2019. Another option is that you could just remap inside Revit to the actual location.
NOTE: By default, Program Data is hidden, so if you can see the program data folder you need to turn it on. Click Start > Control Panel > Folder Options > View tab, and then “Show hidden files, folders, and drives”.