I would like to amend my accepted tax return. The online link to amend it through TurboTax leads to an unreachable data file. Is it just currently unavailable?

The amended 1040X don't be available until Feb 20.

The federal amended tax return, Form 1040X, will not be available until on or after 02/20/2020

An amended tax return, Form 1040X, can only be printed, signed, dated and mailed by you to the IRS, it cannot be e-filed. The IRS will take up to 4 months to process the amended tax return. Wait until you have received the tax refund or the taxes owed have been paid and processed by the IRS before amending the tax return.

On the Tax Home web page when the Form 1040X for amending a 2019 tax return becomes available there will be a link - Amend (change) return

To add to what DoninGA and VolvoGirl wrote, I've included an availability form below. Please keep an eye on the dates because they can change.

IRS forms availability table for TurboTax individu ...