Silicon Valley venture capital money. They also got an investment from Audi itself. Many of these tech startups burn through millions and lose a ton of money because they got a ton of venture funding without a realistic plan for making money. Uber still hasn't ever made a profit as far as I know.
These types of companies either get bought out or shut down suddenly. In this case, my guess is that Audi is using Silvercar as a testing ground for their subscription service.