Yes it is throwing an error for that part.
The part is a large threaded boss for attaching high current cable lugs to. Part number: B4A-PCB-RLM. IHI - THT - B4A-PCB-RLM-KIT Ring Lug Mount Block
Datasheet for the mechanical footprint. https://lugsdirect.com/PDF_Webprint/B4A-PCB(-45)-CUTOUT-LAYOUT-FOOTPRINT.pdf
Here is the library i have been building up if you want to take a look:
https://github.com/LonghornEngineer/LHE-LibrePCB-Part
Is there a better way to design this in Libre? I am really excited that the pad editor supports plated slots from the get go, no silly workarounds! 