the split tool can be tricky. Easiest to use if you place a line first.
Then the split tool can use line for the split operation.
1) Select slab
2) Engage the split tool
3) Click on the line you made for this purpose (or any preexisting line)
4) click to one side of line
5) that should be it.
HOWEVER:
Your goal is to have different materials in each room.
Use the zone tool for each room. The zone tool allows you to do many useful things:
A) change materials per room
B) list doors per room number (ie zone number)
C) have room areas at hand
D) scheduling of zone attributes
E) show different fills per room
OR:
Use the Interior Wizard Add-on.
You should run a search (box at top right of Forum page) on "Zones".
lots of useful discussions. Like
Here,
here and
here. tons more . . .
Snap
PS you could also keep main slab for structural purposes etc. and place
thin "finishing" slabs in each room. Look also at the magic wand. This tool
will save time and is a real miracle worker.