I'm using both VS17 & VS19, I've opened the Android SDK (as administrator after it didn't install otherwise) and I don't have platforms higher than 27:
I've tried to install all the relevant Build Tools via the Tools tab but the max platform version is still 27.
First, have a check with the visual studio version whether is the latest version. Such as Visual Studio 2019 is 16.8.3
.And check the Tools of SDK Manager whether install the relative latest version of Android SDK Build Tools :
Second, in the latest version of VS 2019, you could choose choose Full List as follows:
Last, you could have a check with network whether can connect to the server of Andoid SDK well.