This isn’t an actual question but I just wanted to put this here for any future students that want to install 2D Tilemap Extras for the 2D course. Placing it here because I didn’t know where else to put it 
I got an error message when trying to install this package through the Package Manager list (previews enabled). To solve this I got the source code from github.
The way that works to go to github where the source code is:
Then choose the branch that corresponds to the version of Unity that you are using (mine was 2020.3).
(you can choose the branch where it first says Master)
Click on the green button that says Code and copy the link provided.
Go to Unity Package Manager and in the top left click the + icon followed by ‘Add package from git URL’
Paste the link there. When I did only this I got the error stating that ‘ReordableList does not contain …etc.’ To solve this issue don’t add the package just yet.
Behind the link type a # sign followed by the version of unity that you use. In my case that was #2020.3. The link should look something like this: https://github.com/Unity-Technologies/2d-extras.git#2020.3
This should install 2D tilemap extras in your project and prevent this error message from popping up 
Hope this is of use for anyone in the future!