@Xaphane7 It's probably that your save is overloaded or a bit corrupt, although that doesn't mean you can't fix it. Since you mentioned NRaas mods, have you run a MasterController town reset (NRaas > MC > Town Options > Reset Everything) yet? Hopefully your NRaas mods include ErrorTrap and Overwatch as well.
If the MC reset doesn't make a difference, try turning all your graphics options down to the minimum, at least while using Edit Town. Be sure to change the settings at the Main Menu and quit to desktop before loading the save. This can help on the margins: if the game can't quite handle the assigned tasks, maybe it could on the lowest settings.
If the town currently has inclement weather or snow on the ground, you can also try temporarily changing it. Bring up the cheats console (crtl-shift-C) and enter "testingcheatsenabled true" without quotes, then shift-click on the ground and select sunny weather, plus no snow cover if applicable.
Placing new lot grids in Edit Town can be difficult even without any other issues, so I'm not sure what to suggest except to keep trying. Sometimes it can help to zoom in (with the scroll wheel) to get more control over exactly where you're placing the lot. And sometimes there's a wild animal in the area and the game won't let you place anything on top of it. MC resets are very convenient here.
Any time you do get the change you want, it's a good idea save, or better "save as" and rename the save, before trying anything else. That way, if the game crashes later, you don't have to start from the beginning.
And any time you quit or get a crash, be sure to delete the five cache files in your Sims 3 folder in Documents\Electronic Arts. For reference:
- CASPartCache.package
- compositorCache.package
- scriptCache.package
- simCompositorCache.package
- socialCache.package
Sometimes working with an older save is a lot more trouble than than it seems to be worth, but hopefully you'll find a way to get through this and go back to actually playing.