Every time i run the script i got error "Error executing the grid expression, run time error, Failed to execute (Slope). " ? What is wrong with in code ?
When i run the Slope tool in ArcGIS it's give me the same error, but when i run the script in ArcGIS python Command prompt it runs successfully what is the problem, i don't understands ? 
import arcpy
from arcpy import env
from arcpy.sa import *
env.workspace = "......."
if arcpy.CheckExtension("spatial") == "Available":
arcpy.CheckOutExtension("spatial")
outraster = arcpy.sa.Slope("elevation", "PERCENT_RISE")
outraster.save("slope_per")
arcpy.CheckInExtension("spatial")
else:
print "Spatial Analyst Liscence is not avaliable"