I've been using TT for Mac desktop for decades, and I'm accustomed to the quirks, so I'll try to help again. If you're sure you have the accurate cost basis amount, just type it on the first entry form and keep the "Cost basis is incorrect on my 1099-B" box checked. Then when you step through it'll basically be confirming that you are sure of this holding time frame. Since you have no acquisition date reported, if you're sure that every share you sold (including any reinvestments) are all from over a year ago, reaffirm that it's a long term gain.
Then if TT makes you do a loop-dee-loop just fix what it "forgot" as far as the cost basis you entered and the 'Various' as the date and step through again to confirm. It should stick. It's basically just entering it on your Form 8949 as an "E" type sale which is a long-term transaction where no cost-basis was reported by the broker. As long as you have your own record for your purchase(s) you're good to go.
