System error when trying to place a Custom Venue from my library

lurveislurve​ and all other posters on this thread.  Try this if you still have the original lot you built.  No guarantee that it will work but you never know.  I've had it work once or twice though.  Clear everything off of the first perimeter tile all the way around the lot.  If you are able, flatten the lot.  The goal being that the last tile is flat when compared to the world around it.  One thing I have noticed is that sometimes if you have a MOO placed or rotated object on a perimeter tile, it will cause problems.  1 tile objects usually get deleted but bigger objects sometimes block you from saving or placing the lot.  Another thing that you can check is your roof overhangs for extending into that last tile or even overhanging the edge of the lot.  Pull the eaves in as far as you can before saving the lot.  If the lot places then, you can pull them back out and/or replace anything you deleted on the perimeter tiles.

Another thing that may be happening is something I've seen in the SM apartments when you try to place a renovated apartment over them in one go by using the delete a tile between rooms so the apartment will place complete rather than separate rooms.  Sometimes, you have to delete everything including 1 tile of any connecting wall, on the tiles on the last perimeter tile against another apartment to get it to place.  Something like that might be going on here.  If the lot will place for you, everything will probably be one tile off in at least one direction.  I have no idea if this will work as I have no way to test it.  

Another thing you can try is to save the build with no furnishings.  I'm assuming most people build like I do, constantly placing and deleting a ton of objects along the way until I get everything like I want it.  Sometimes, a deleted object leaves something behind blocking tiles and causing other problems including corrupting a lot.  Platforms are really good for doing that.  So, one way around that is to not save the building with the furnishings.   There's a couple of ways to do that, but I find that moving a sim in, then moving them out and keep the furnishing works best.  That tends to clean out all the invisible objects.  Then all you have to do is place the lot where you want it unfurnished and then redecorate.

Hope this helps.