tax2016 to txf conversion seems to be related to export of TurboTax files to TXF format, probably for the purpose of exchange of data between different platforms or software. Everything points to the fact that the only software that can work with .tax2016 is TurboTax and most likely only in the 2016 edition or perhaps later.
Unfortunately, we do not know about any specifics, so if you have issues with TurboTax files, it would be perhaps best to contact their support.
Open TAX2016 file Open TXF file