WCMO is that Europeans “can’t” make a large flat white.
Me: A large flat white please.
Them: I am sorry we can only do medium flat whites.
Me: May I have a large latte with no foam then?
Them: Of course, here you are.
Breathe Jessica. Breathe.
Like the classic joke:
"A coffee without milk please."
"I'm sorry, we ran out of milk, but I can make you a coffee without cream."
Don't know where you are in Europe, but most Europeans aren't aware of the range of coffees that we have, and most likely wouldn't make or offer them. Depending on where you are in Europe, you might have to settle for a cafe creme, a white coffee, a latte macchiato, a cafe renverse or a cafe latte (some of those are variants on a latte rather than a flat white).
We (Australians) think we have some of the best coffee in the world, but if we're really honest, that's like saying the Americans have some of the best pizza in the world. Coffee, quite pure, still has its heart in Europe, Africa and tropical Latin America; definitely not Australia. When I travel in most countries in Europe, I try to keep my coffee simple (even just an espresso).
A similar experience asking for a croissant with cheese and ham, along the coast from Genova recently. (Not for me, by the way.)

“No. Only plain croissant. I can make a panini with cheese and ham.”
“But you have the croissant, the cheese and the ham?”
“Yes, but only bread with cheese and ham.”
“Thank you. I will have a panini with cheese and ham and a plain croissant on the side.”
“Va bene.”
Most Europeans are taught from a young age to accept what you are given to eat. For example, at a French meal, it is considered very rude to decline or scrape aside (eat selectively) what has been served to you; the only main exception is allergies (which you probably should have told your host in advance if you knew you had the chance).
The culture of substitutions of menu items, no matter how minor or otherwise, was borne mainly in the Americas (basically a Western thing). Just another notch on the list of things that Americans do that annoy the living daylights out of Europeans. (Let's not get on Starbucks orders, especially those that make it to TikTok)
That also said, Europeans rarely (read: I've never seen it) eat croissants loaded with ingredients (like a sandwich). Typically they are sold and eaten plain.