How do I access the Order object in an Invoice form

When you are in the invoice form, you do not have access to the Order object.

To get to an order, you would need to find it through invoice_items that is part of an order.

An invoice can be related to an order in 2 ways.
If the invoice is created from an order without shipping it, you would access order by accessing any of the invoice items and then the order_item_invoicings records.

For example
{% assign first_line = Invoice.invoice_items | first %}
{%assign first_invoicing = first_line.order_item_invoicings  | first %}
{{ first_invoicing.order_item.order.order_number}}

If the invoice is created from an order by a shipment, you would access order by accessing any of the invoice items and then the order_shipments records.

For example
{% assign first_line = Invoice.invoice_items | first %}
{% assign first_os = first_line.order_shipments  | first %}
{{ first_os.order_item.order.order_number}}