Unity run out of memory

I started working on an older games (I think i startet at 5.6 now using 2018.3) and tryed out the terrain-system with multiple terrains working together. Before in Unity 2017 i tryed on large terrain, but i t had a error with building. Now there are multiple errors while baking, it took like for 2 hours yet and going on (stuck at visibility, around 550 jobs to do). It said that it Out of memory loading input data and my Questions are:
-What exactly can cause this?
-What can i do (how to optimize the scene)?
-Is it later bad for the game-performance?

//Update while writing: Total memory (physical and paged): 17886MB

I have 16 GB of RAM and Unity uses about 6 to 8 GB

 

If you have 16GB of RAM, why does the error say you have under 18GB of memory including your page file? Out of disk space?

 

HI, I solved same error on Unity 2019.4.14f1 with a windows 10 pro i7 witth 16Gb ram on a "big" scene (2h14 backing) by switching Ligthmapper to "Progressive CPU" (because i noticed that former "Enlighten" Lightmapper was flagged Deprecated. Hope this helps