Unable to 'Propose Going Steady'

At the risk of asking a silly question first (sorry), are the sims Romantic Interests with each other? Steadies are the second level of romantic coupling, not the first.

Sometimes these things do pose a mystery. It might be the ex relationship that is still getting in the way and the Clean Slate LTR might help clear that. Have you tried playing the other sim as active (just as a test, you don't have to save after doing that) to make sure they don't really already have a partner?

Are mods in play? There might be one interfering or some that can help. NRaas MasterController for example can be used to just set the romantic coupling status where you want it to be as sort of an all else has failed kind of approach, if such a status is viable. Another of our mods we are not allowed to discuss here in debug mode can tell us precisely why the Ask to Go Steady command is not appearing, but we would have to help with that over at NRaas.