It sounds like what you're doing wrong is not understanding the law.

Did she live in it as her primary residence for 24 months during the last 60 months she owned it?

If she rented it out since 2018(when renting started?), rented 2019, 2020, and part of 2021(when sold?), she might have exceeded 36 months since she last lived in it.

If she sold it in the 37th month after renting started, SHE GETS ZERO EXCLUSION.