Changing an html website to WordPress and missing the Parent Theme

I have been trying to change my HTML website over to WordPress. When I load the new theme, it says it is installed but that the Parent Theme is missing, please install. How do I do that? I have copy and pasted all the code into the files to transfer it to WordPress. These files are header.php, footer.php, style.css, and sidebar.php. This website was designed years ago and was HTML. I feel like I am really close but I don’t know how to create the Parent Theme when it was created outside of WordPress. Please help if you can.
Best,
Helen