You can't get standard car insurance for a vehicle you don't legally own or have a documented insurable interest in. Insurers require proof that you would face financial loss if the car were damaged, usually shown through vehicle ownership or registration. Without that, the insurer sees you as a third party and views the arrangement as a red flag.
Some insurers offer non-owner insurance, but that only covers your liability, not the car itself. If you need coverage, you'll need to explore other options like getting added to the owner's policy or the vehicle title.