Quick answer
Generally no. Umbrella carriers typically require an underlying auto policy meeting minimum liability limits, often $250,000 to $500,000, as a condition of coverage, precisely because umbrella insurance is designed to sit on top of that underlying policy, not stand in for it.
If you don’t own a vehicle at all, some carriers offer alternative underlying requirements, but the general rule holds: umbrella coverage needs a qualifying underlying policy beneath it to function as intended.