@missy28
I don't get the option for the master invention too. I confirm that this is a common bug. It isn't a computer related problem like EA stuff tries to point that out in each support ticket of the people.
EA simply doesn't care. They even fixed this not in their recent patch (this month).
I tested it and re-installed the game, applied all patches on a newest computer with windows 7 64 bit installed and run the game, created a new Sim, made him an inventor, waited the opportunity,
brought all requested materials in: piece of palladium, heart shape cut pink diamond, 10 life fruit. I got in the last step a life core, that's such a weird looking item in your Sims inventory.
I went to my Sims workbench, bought 1000 units of scrap to ensure an option should appear. NOTHING! No Simbot construction option there, just nothing. No grayed out option, nothing.
The option pie menu string should be: "Make mystery invention" in the main pie menu of your workbench. The option is missing. THIS IS A NEW BUG. I'm fed up with EA recent bugs, it's just annoying.