Welcome to Unity.
Please use code tags whenever you want to insert code. It makes reading code much easier.
Also, please post the complete error message and describe a little more what you’ve done.
Are those buttons assigned in the inspector?
The conversion cannot be done because that particular find method returns a GameObject. A GameObject is not a Button, but it might have a button component attached. You’ll need to call GetComponent on the returned object. However, it’ll be easier if you just set the buttons in the inspector.