Did you enable 

• testingcheats 

and then

• modifyrelationship <FirstNameSim1> <LastNameSim1> <FirstNameSim2> <LastNameSim2> x LTR_Friendship_Main

I'm having the same issue ever since the latest July update. When I type in "testingcheats true" it doesn't show up in the window that it's been enabled as it used to. It seems to be working, just not showing up after hitting enter. I'm not sure if that has anything to do with the relationship cheat not working, but no matter which Sims I've tried, the cheat doesn't change their relationship. Any advice?  Thank you!