I found 3 issues here.
First, Linesensor3 isn’t an object. You forgot to capitalize it to match what you created, LineSensor3.
Second, value is a function and needs the parentheses and parameters to be called. it should look like this: LineSensor1.value(pct)
pct is for percent, change it for your own use.
Finally, if statements need brackets to define the scope of the statement. Assuming you want to stop the motor when the if statement is true, your main function turns into this:
int main() {
// Initializing Robot Configuration. DO NOT REMOVE!
vexcodeInit();
while (true) {
LeftMotor.spin(directionType::rev, 50, vex::velocityUnits::pct);
RightMotor.spin(directionType::rev, 50, vex::velocityUnits::pct);
if (LineSensor1.value(pct) < 1000 && LineSensor3.value(pct) < 1000){
LeftMotor.stop(brakeType::coast);
RightMotor.stop(brakeType::coast);
}
}
}