When you call function stripes(), you always use 1 as parameter.
Then, SpeedX receives 1.
So before any checks, n = n+SpeedX; happens.
After that, it's irrelevant changing the sign of SpeedX!

You might try to remove the semicolons

The extra semicolons after curly bracers are inoffensive.
Instead, I've removed the curly bracers themselves in my version! 

Here's my take on it:  (declared spdX as a global variable along w/ n)

int n, spdX = 1;

void setup() {
    size(768, 576);
    frameRate(12);
    smooth();
    stroke(0);
}

void draw() {
    background(255);
    stripes();
}

void stripes() {
    n += spdX;

    // this should reverse the motion, no?
    if (n<-50 || n>50)   spdX *= -1;

    for (int i=-20; i<width+50; i+=10)
        line(i, 0, n+i, 30);
}

thank you,but the problem is i want to call different instances of stripes() with different speeds
that's why i tried to put void stripes(float speedX) but that doesn't work
how do i proceed in this case?

It won't work, because you would need a n for each speed, too.
In this case, you can wrap the stripe() function in a class, so each instance has it own n, speedX, and perhaps posY.