I have an inventory adjustment with two lines. The period is open. Why would I be able to edit the estimated unit cost on one but not the other?

Is one a negative adj qty? You can only change the unit cost on positive adjustments...negative adjustments use the item Avg Cost

got it...so theoretically, the "avg cost" would only be 0.00 if it was initially added via inventory adjustment at 0.00 or received in on a PO at 0.00

Correct