The error occurs when WordPress tries to call wp_get_theme() but the function is not defined. The function lives in wp-includes/theme.php, which is loaded by wp-settings.php. If the core file is missing, corrupted, or not included, the call fails and PHP throws a fatal error.
Typical triggers include an incomplete update, a theme or plugin that runs before core files, wrong ABSPATH, or file‑system permission problems that prevent the core file from loading.