When you have installed your theme, and you can see it under appearance, but you can not activate it. There is a good chance your setup does not meet the requirements.
The buttons will show like:
Your theme will have requirements, specified in the style.css for your theme.
/* Theme Name: Twenty Twenty Text Domain: twentytwenty Version: 1.1 Requires at least: 4.7 Requires PHP: 5.2.4 Description: Our default theme for 2020 is designed to take full advantage of the flexibility of the block editor. Organizations and businesses have the ability to create dynamic landing pages with endless layouts using the group and column blocks. The centered content column and fine-tuned typography also makes it perfect for traditional blogs. Complete editor styles give you a good idea of what your content will look like, even before you publish. You can give your site a personal touch by changing the background colors and the accent color in the Customizer. The colors of all elements on your site are automatically calculated based on the colors you pick, ensuring a high, accessible color contrast for your visitors. Tags: blog, one-column, custom-background, custom-colors, custom-logo, custom-menu, editor-style, featured-images, footer-widgets, full-width-template, rtl-language-support, sticky-post, theme-options, threaded-comments, translation-ready, block-styles, wide-blocks, accessibility-ready Author: the WordPress team Author URI: https://wordpress.org/ Theme URI: https://wordpress.org/themes/twentytwenty/ License: GNU General Public License v2 or later License URI: http://www.gnu.org/licenses/gpl-2.0.html */
If your setup (php, WordPress) does not meet the theme requirements you will see the buttons as in the first image. So upgrade your setup.