In most cases, 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, typically demonstrated through vehicle ownership or registration. Without that, the insurer sees you as a third party and may view the arrangement as a red flag.
A few insurers may offer non-owner insurance, but that only covers your liability, not the car itself. If you need coverage, you may have to explore other options like getting added to the owner's policy or the vehicle title.