Hi @armanoj 

Try this:

1: Create the Script Include

• Navigate to System Definition > Script Includes >Click New

• Name: checkApprovalAttachment
• Client callable: Check the box (true)

var chkApprovalAttachment = Class.create();

chkApprovalAttachment.prototype = Object.extendsObject(AbstractAjaxProcessor, {

    hasAttachment: function() {

        var approvalId = this.getParameter('sysparm_approval_id');

        var grAttach = new GlideRecord('sys_attachment');

        grAttach.addQuery('table_sys_id', approvalId);

        grAttach.query();

        if (grAttach.next())

        {

            return 'true';

        }

        return 'false';

    },

    type: 'chkApprovalAttachment'

});

2: Create the Client Script

Navigate to System Definition > Client Scripts>Click New

• Name: Mandatory Attachment on Approval
• Table: Approval [sysapproval_approver]
• Type: onSubmit
• UI Type: All

function onSubmit() {

        var state = g_form.getValue('state');

    if (state == 'rejected') {

        return true;

    }

    var ga = new GlideAjax('chkApprovalAttachment');

    ga.addParam('sysparm_name', 'hasAttachment');

    ga.addParam('sysparm_approval_id', g_form.getUniqueValue());

    var response = ga.getXMLWait();

    var result = response.documentElement.getAttribute("answer");

    if (result == 'false') {

        g_form.addErrorMessage('An attachment is required before you can submit your approval.');

        return false;

    }

}