Sales Order > Create PO Option Unavailable for Item

We found a Lot Numbered Inventory Item in our account where the Create PO checkbox is not available on the Sales Order; it's basically a greyed out box. This doesn't appear to be a new issue as I checked about 10 transactions over the past several years and across multiple subsidiaries, and there are no PO's created for this item line.

Figure it must have something to do with the item setup, but I checked 4 different SuiteAnswers articles and found all of the solutions made no change:

SuiteAnswers articles: #65289 (we have a preferred vendor and purchase price), #19584 (we have other items that do not have either of these boxes checked, but can still select Create PO), #96939 (see response above for #65289), #92794 (I tried this just to see if it might have any effect, but no changes).