A few things:
1) Please copy and paste the XML source and use the codetags as noted here
https://coderanch.com/wiki/659781/Code-Tags
2) You do not need to use the CardView for these beginning tests/steps. Maybe later on in your app development you will need this control/view, but not right now.
3) The size of the CardView could be an issue, depending on you phone. This is most likely the issue that you are experiencing.
4) I suggest that you go over this tutorial
https://developer.android.com/training/constraint-layout/index.html
Here is a basic layout that will work on nearly every device
If you replace what you have with the XML from this post
you should be fine.
NOTE: Make sure that you have the correct tools:context value. In this example it is myapplication.MainActivity, but in your it is something else.
This XML is does not have the CardView control in it. Feel free to add it in whenever you want.
I encourage you to edit the XML and change the
layout_constraint tags to see what happens.
Right now the text is the center of the screen, but with a few quick edits you can change that.
You should become somewhat familiar with the XML happening behind the scene, or at least aware so that you can make drastic changes to your view with a few edits.