If you can’t get any of the cheats to work for you (and I never have) I’ve found the best way to earn LP quickly is to do ghosthunting.
Gather as many sims as you can together in one house, and then place a couple of haunted items around the house. Then get all of your sims searching for ghosts (by clicking on your haunted items).
For an added chance of finding LP relocate pets to the same house so they can find treasure while your sims hunt.
When you catch a ghost you’ll get simoleons or XP as a reward, occasionally it’ll be LP. When you get the 4th set you’ll get 3 LP, and when you get the full set you’ll get 500,000 town value (which automatically gives you 5 LP). The more sims you have ghosthunting, the quicker you’ll be able to complete the set. As your sims level up in the hobby they’ll also earn XP and LP.
In about 25 minutes of ghost hunting I was able to get 10 LP, with 3 sims hunting.
You can repeat as many times as you want to get as much LP as you need. It’s the quickest way I’ve found to earn LP.