Query regarding ITR -2

I am filing the ITR (FY 2018-19) for my mother. She is a housewife but some sale and purchase of property is done on her name in that year. During the transaction some cash has been deposited inher account as cash and same is shown as SFT entry in her 26AS. While filing the ITR i have to mention the PAN of Buyer but i am UNABLE to get the same from him/property dealer and system is not accepting the ITR-2 File and showing error. Pls help. Need assistant urgently.

Thanks in advance !