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Hi, I'm Mike and I'll be assisting you. Did you test at the main breaker for the sub panel?
That's exactly what happened. Your feeder should be replaced with 4 wires. 2 hots, 1 neutral and 1 ground. All conductors should be in conduit.
You need 4 wires and a ground rod for sub panels. That's code.
Anytime, you're welcome.