How to adjust the lower Limits för a room?
The error message is telling you exactly what the issue is.
So what you actually need to know is how to raise the computation height, NOT "How to adjust the lower limits for a room."
Here is a help topic on how to adjust the computation height. It is a value derived from a setting on the level the room is hosted to.
http://help.autodesk.com/view/RVT/2018/ENU/?guid=GUID-2945C619-2AE6-455D-823C-E4F6AE759B8E
