I had this happen to me this past week. I had already requested a PP upgrade on an itinerary last week. For grins, I wanted to know what United was charging for a ToD upgrade. But when I selected this option in my reservation from the app, I was told:

"This reservation is not eligible to be upgraded"

I presume that this was because I had already submitted an upgrade request ... which may or may not be relevant to the OP.