For the 2020 tax year, this is not a "bug" but is an IRS e-file requirement. The IRS will reject the return this year if negative numbers are listed as the basis. So, TurboTax has been programmed to comply with this. The reason was the same last year - this is the IRS preferred format - but last year they relented because the IRS would still accept those returns.
This year, the IRS will reject them, so TurboTax is giving you the error to avoid the e-file rejection.
You can file with losses, you just need to adjust the entries so that there are not negative numbers.
