job_bm_job_submit_errors

add_filter('job_bm_job_submit_errors', 'job_bm_job_submit_errors_29072019', 90,2);
function job_bm_job_submit_errors_29072019($error, $post_data){

    if(empty($post_data['custom_form_field'])){

        $error->add( 'custom_form_field', __( 'ERROR: Field is empty.', 'job-board-manager' ) );
    }


    return $error;
}

Remove errors

You can remove error message if you remove required fields.

add_action('job_bm_job_submit_form','remove_some_fields_20200512', 2);

function remove_some_fields_20200512(){

    remove_action('job_bm_job_submit_form','job_bm_job_submit_form_company_name',45);
    remove_action('job_bm_job_submit_form','job_bm_job_submit_form_location',45);
    remove_action('job_bm_job_submit_form','job_bm_job_submit_form_address',45);

}

add_filter('job_bm_job_submit_errors', 'job_bm_job_submit_errors_29072019', 90,2);
function job_bm_job_submit_errors_29072019($error, $post_data){

    // Get errors
    $errors = isset($error->errors) ? $error->errors : array();

    // Remove errors
    unset($errors['job_bm_company_name']);
    unset($errors['job_bm_location']);
    unset($errors['job_bm_address']);


    // Assign/update errors
    $error->errors = $errors;

    return $error;
}