NEED HELP!

THE FOLLOWING ERROR OCCURED WITH THE BELOW CODE

while(1U)
{
P1.1 = 0;
}

ERROR: "E0520020:identifier "P1.1" is undefined"

I ALSO TRIED THE BELOW CODE
while(1U)
{
P1_1 = 0;
}
SAME ERROR! CAN ANYONE HELP ??????