if you want to pay TT fees, you can prepare your 2016 return online but because of the changeover from 2018 to 2019 you may have to wait until some time in December. contact support about the 2016 issue. there can be many reasons for this. have you tried a complete unistallation of 2016, reboot, and try to reinstall from cd? some ops may post other suggestions. you can file 2017 and 2018 without filing 2016. e-filing any of these years is closed. mail each year separately following TT mailing instructions. use a method where either you can track them or get proof of delivery. be sure to attach any withholding documents.
as for 2016 since the earliest due date was 4/15/2017 you have until 4/15/2020 to claim refund. if the IRS received an extension for that year, you have until 10/15/2020, but don't bet your refund on this.
