Your site does not have any default theme.
You probably encountered the following warning in the Tools -> Site Health of your WordPress Dashboard: "Your site does not have any default theme". Usually, WordPress recognizes one of its themes as default, such as Twenty Twenty-One, Twenty Twenty, Twenty Nineteen, etc. However, if you delete the predefined WordPress theme and want to keep your custom theme, you have the option to set your own theme as default in the file wp-config.php.
Adding the code
To define a default theme, insert the following line in the wp-config.php file, just before the line that says /* That’s all, stop editing! Happy blogging. */
define(‘WP_DEFAULT_THEME’, ‘the name of your theme directory’);