Source

File: wp-includes/rest-api/endpoints/class-wp-rest-plugins-controller.php

protected function is_filesystem_available() {
		$filesystem_method = get_filesystem_method();

		if ( 'direct' === $filesystem_method ) {
			return true;
		}

		ob_start();
		$filesystem_credentials_are_stored = request_filesystem_credentials( self_admin_url() );
		ob_end_clean();

		if ( $filesystem_credentials_are_stored ) {
			return true;
		}

		return new WP_Error( 'fs_unavailable', __( 'The filesystem is currently unavailable for managing plugins.' ), array( 'status' => 500 ) );
	}