The new feature in SW2021 that has the relations pop-up at the cursor for sketch elements is a great enhancement, and has been requested by many users for many years. I appreciate the efforts to get it implemented. However, the unfortunate fact is that if you select a sketch element first, and then a plane, model edge, etc belonging to a model, it doesn't appear. You can still go to the property manager to add the relation, but the pop-up isn't there unless you've selected the sketch element last.
I'm having trouble figuring out why anyone would think that the order of selection should determine whether or not the pop-up would appear. If someone can explain it to me that would be great. If not, I'm going to add this to the list of great enhancements that I believe were poorly implemented.