letter suggests Turbotax help desk .. LT loss to be entered on 1040, form 8949, part II ; numbers dont match and could not add/ enter 8949 (copy x)
I would start with reaching out to the fiduciary and asking them to give you the Schedule K-1 that is marked "final".
You might also see what was reported to the IRS under your Wage and Income transcript. You might get some of the information regarding the loss from the transcript. Here is how to do that.
You can also enter the information into TurboTax as if you received Form 1099-B. Although, if you are supposed to have Schedule K-1 and don't report it, it may generate a letter from the IRS.
Another thought about your situation is that the letter you received may be directing you to report the basis loss and that would go on 8949. But you would still need to report Schedule K-1.
A grantor letter is often issued in lieu of a K-1 form so you will enter the information into the program as if you did get a K-1 however you cannot use the K-1 entry screen ... you must enter the individual income items in the correct placed in the interview. For instance if you have interest, dividends or cap gains you need to enter them in the 1099-INT, 1099-DIV and 1099-B sections as if you had gotten these forms.