Got an overpayment notice, what now? after 14 years as a hotel front-desk agent

The agency says I reported earnings incorrectly, but I think I followed the instructions. I do not usually post, so bear with me. I cannot afford to repay it all at once. Who should I talk to first?

I asked for a waiver and a plan in the same call, and they gave me both forms. It was less scary than I expected.