First, I suspect Turbotax is prioritizing issues affecting new filers rather than the amended return.
Second, Turbotax has a habit of releasing updates late in the stated day. (I have suggested that they should make the official day the day after, in that case, but they didn't take the suggestion.) So it may be available later tonight or tomorrow morning.
Third, an amended return is a complete full tax return with all required schedules and attachments containing the corrected information, PLUS the form 1040X cover sheet that summarizes the changes. If you only mail a 1040X the IRS will send it back without processing it. If you believe you would rather generate your entire new correct tax return on your own, go ahead. There is also no rush, you have 3 years to file an amended return to claim an additional refund, and if you owe additional tax, there will be no interest or late fees as long as you mail it by April 15.