How to make date field as 30 days from today

Hi @amal_31845,

Not possible to set it in default.

You should apply the client script for that.

Please check the reference:

frappe.ui.form.on('Sales Order', {
    refresh(frm) {
        frm.set_value("delivery_date", frappe.datetime.add_days(frm.doc.transaction_date, 30));
    }
});

Thank you