Employer Voided Agreement Seven Days After Receiving

Location: New York, NY
I recently quit my company on a written agreement that I would take 2 months severance. My final day was September 15th, 2025. I received the Severance Agreement on September 23rd.
A clause for acknowledgements stated that "Employee agrees that Employee has been paid what Employee is owed". This clause included expense reimbursements for which I was waiting on a final expense reimbursement. I spoke with the PEO who contacted my company and stated that it would be processed on September 19, 2025. The expense reimbursement was never processed.
The morning of September 23, I got an email saying the severance was voided. However, there was a clause that I had 21 days to consider the agreement.
I've contacted my HR through multiple emails, and they have been ghost with no response. Do I have any legal recourse to sue in the case that they did not uphold the contract?
I am not above the age of 40, so the Age Descrimination in Employment Act which includes the 21 days to consider clause doesn't seem to apply to me.
I was also not given a specific date to sign by or told of any expiry date other than the 21 day clause in the agreement. They just voided it after 7 days of me receiving it.
Edit: Including more context
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