Selling my house. Do I need my own septic for the buyer to qualify for a mortage?

I'm trying to sell a house that shares the neighbors septic and the buyer says she can't get a mortgage because it's not my system and a bank won't lend with no guarantee their is proper plumbing. Do I need to have my own system installed? Should I try to get the neighbor to somehow change the deed to make it a co-owned (and thus both responsible for) system? Would be great not to have to drop 10k. I bought the house cash so never found out until now! Thanks everyone for your help.

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