The tape measure tool has (as do most other SU tools) multiple functions which it can do. and these get implemented by toggling the ‘option’ (/‘ctrl’) key on your keyboard while the tape measure tool is active.
So, the first thing to check should be what state the tape measure tool is placed in… and that’s typically reported down on the bottom row of the SU window L/H side of the Value Control Box.
Anyhow, The tape measure tools alternative functions are to create guide lines, and guide points… and while placed in one of the guide modes, one might tend to think something is wrong if they’re only expecting to experience a measurement function.
It’s worth opening up the ‘Instructor’ Window, while your working through this, so you can see what all of the alternative functions are for all of the tools, while your getting the hang of things.
Aside from providing a basic summary of what the active tools do, you’ll also get links to SU’s Knowledge Base Articles for each tool…