@Maria
When you download an extension from the PluginStore, using your browser, there are several possible outcomes, depending on how you have configured your browser's options...
The RBZ file that is downloaded when you click the button can be handled by the browser in several ways.
For example if RBZ files are incorrectly set up to open with SketchUp, or any other app - like a decompression software, the the download will fail.
Of if the browser is correctly set up to save the RBZ file onto your computer, then there are two possible alternatives.
It might auto-save it to your Downloads folder, or it might prompt you in a dialog to choose a folder in which to save it...
In both of these cases you should now have an RBZ archive on your computer.
You now need to install it into SketchUp.
In SketchUp use the menu item to open the built-in Extension Manager dialog, then use the [red] button to Install...
Find the RBZ file and SketchUp should now install it for you automatically - extracting the files from the archive and copying them into the correct location[s] in your Plugins folder...
If you have problems like this, it can be down to incorrect permissions, and these usually result from SketchUp being incorrectly installed...
It's possible to Repair the installation [without first uninstalling and starting again]...
To do this - login to Windows as your normal user-account.
Ensure SketchUp is not running.
Find the SketchUp installer's exe file - usually in your Downloads folder.
Select this exe file's icon, right-click > context-menu > "Run as administrator"
When prompted choose "Repair".
When it completes restart SketchUp.
This operation should reset SketchUp and your user-account to have the correct permissions.
Installing any complex app in any other way will lead to weird and unpredictable problems.
So never double-click on an installer's exe file to 'Run' it - even if your user-account has admin powers it is not the same as using 'Run as administrator' as exampled above...