Gtag - basic 'purchase' event not firing

I have the following event setup to fire whenever a user on my site makes a successful transaction:

window.gtag("event", "purchase", {
    id: new Date().getTime().toString(),
    value: 59.97, 
     currency: "USD",
});

I picked the event up straight from Googles enhanced ecommerce documentation:

https://developers.google.com/analytics/devguides/collection/gtagjs/enhanced-ecommerce#measure_purchases

No idea why it's not picking up my recent sales?

Note: I have enabled enhanced ecommerce in my GA admin.

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