Hi @VarshiniG ,

If Attachment variable related solution is not accpted, last option to 

Create a onSubmit client script

Sample code:

function onSubmit() {
    var sysIDrecord = g_form.getUniqueValue();
    var validateAttach = new GlideRecord('sys_attachment');
    validateAttach.addQuery('table_name', 'sc_cat_item_producer'); 
    validateAttach.addQuery('table_sys_id', sysIDrecord);
    validateAttach.query();
    if (!validateAttach.next()) {
        if (confirm('Please attach a document')) {
            return false;
        }
        else {
            return true;
        }
    }
}