Urgh what a horrible pressure on parents to have to buy a CAR for a child's 21st. About 3 or 4 (young) people I know, had their parents buy them cars, but this is not the reality for many.
For my 2, we paid for them to learn to drive. Cost maybe £500 to £600 each, (over 6 to 8 months,) and we spent about a couple of hundred each on gifts on the day too. They had a big party with their mates, which they and all their mates chipped in for.
But a car? No. No chance.
It's an unreasonable request and very grabby.
When I was growing up in the 70's and 80's, I didn't know a SOUL whose parents bought them a car for their 21st. Except one man I knew (in the early 1990's,) who owned a company, employing 50 people, bought his one daughter (an only child,) a brand new car for the 21st.
But this was the only one I knew of. Everyone else bought their own car (and paid for their own driving lessons!)
Your daughter can do the same. And I am willing to bet it is not the 'norm' for all her friends to have their parents buy a car for them. I bet there are more who have not had their parents buy them a car.