Hi Leo,
From the error code, its pretty clear what the problem is in your code.
Line 25
outRaster = rasters / rastVal[rast] * 100
TypeError: unsupported operand type(s) for /: 'list' and 'float'
From your code I'm not sure exactly what you are trying to do, but you are dividing a list by a float and that throws a type error, since rasters = [os.path.join(raster_ws, r) for r in arcpy.ListRasters()], so your variable rasters is a list.
Depending on what you are trying to do, you will probably need to loop through your rasters list to get the actual raster out of it.
If you could make it a bit more clear what you are trying to do, it would be a little easier to help you out, but thats why its throwing an error.
Edit: Read your post a bit more carefully, if you just need the first raster from your rasters list, use an index, otherwise, you will need to loop through them.