I have found that various tools added to my Quick Access Toolbar become unavailable during normal use of Pro 3.6
Some of this has been observed in 3.5.x but 3.6 seems quite feral in this regard.
I've previously commented on the Select tool being greyed out and here is Measure doing the same.
I ran an XY table to point and then zoomed in in the active map where the points were added to measure the offset of the new vs old points - low & behold the measure tool is unavailable.

You can restore it by clicking on the main Map tab of the ribbon and all the QAT tools will activate again.
To see it it is the map not being active I clicked on the layout, other maps, and back to the one I was using and the tools remain in-active. Seems only the activating the Map ribbon fixes it.