You need to complete a value dispute as no investor will pay for repairs prior to closing. Some investors may allow a credit for repairs, but it's typically better to just lower the purchase price as this is more likely to be approved. Work towards getting two bids for the septic system. Do you have inspections showing that the septic system needs to be replaced? If there are additional repairs I'd be looking into getting two contractor bids with itemized expenses and colored photos of the repairs. Put together a CMA and/or provide an appraisal along with the bids. You need to push Fannie to accept a lower value so the buyer has enough room to complete the repairs.
Feel free to reach out to me anytime with any questions as my door is always open.