Fatal error: Uncaught Error: Call to undefined function wp_get_mu_plugins()

The error occurs when WordPress tries to call wp_get_mu_plugins() but the function is not defined. The function lives in wp‑includes/functions.php and is loaded when core files are intact and the site runs a version including it. If the core file is missing, corrupted, or the site runs an older version, PHP cannot find the function and stops execution.

A plugin or must‑use code that calls the function before WordPress loads its core might also trigger the error. Incorrect definition of WPMU_PLUGIN_DIR or file‑system permission issues might prevent the core file from being read.