A few days ago, I stopped being able to right click on my sims to use cheats when testing cheats were enabled. I've logged out/in again, restarted my computer, attempted playing in a clean boot (though I'm not sure I did that 100% correctly) but nothing has helped.
To use the testingcheats options on a sim, you need to shift-leftclick on the sim. This will then bring up the cheats options. Is that what you are wanting to do?
What computer are you using, Mac or Windows PC? Have you changed any accessibility settings on your computer to change the default shift key function and right click function?
Have you enabled testingcheats in the cheat console before attempting to shift click the Sim?
