I don't understand. today i've got an error while trying to save a sales receipt: liability account is required. It never happened before.

Hello Sitkarev

Thanks for reaching out to the Community. Our engineers have advised an  investigation has been raised into this issue and they are working on fixing this now.

In the mean time if you are creating transaction in QuickBooks Online you can  choose in the drop down on  the amounts are box 'No VAT' rather than selecting a code in the VAT box, or if adding from the Banking section add using any code and then edit the transaction and change Amounts are to 'No VAT' again.

Emma