"Reject Code 8960_Hold: This return could not be processed for e-filing due to incorrect or missing information." What does this mean? How do I know what needs fixing?

My federal return was rejected with "Reject Code 8960_Hold" but offers no explanation of what was missing or incorrect. TurboTax just tells me to "Please review the reason for the reject.", but doesn't tell me what the error code means, what is incorrect, or what needs fixing. Please help!