Since you are using TurboTax Online, you could exit TurboTax, clear your Cache and
Cookies and re-try.
If there is a Form 6252 'stuck' in your return, you can also try deleting it. Click this link for steps on How to View and Delete Forms in TurboTax Online.
Or, in Sale of Business Property, check only the box that says 'Installment Sale'. If you show a 'Copy 1' on the next page summary, delete it.