my effect 901 (circle) run as a "8" figure

this is expected behavior.

effect 901 doesn't make your fixture draw a circle no matter what, it sends pan/tilt values.

so in the marked section (going counter-clockwise) this graph says that the pan value decreases while the tilt value increases. the difference of pan in the beginning of this quarter and the end of this quarter is the amount in Scale (by default 25). tilt increases by the same amount.

so if you apply the effect to a moving light that is currently at pan/tilt 0°/0°:
3 o'clock = 25°/0°
12 o'clock = 0°/25°
9 o'clock = -25°/0°
6 o'clock = 0°/-25°

but you were expecting:
3 o'clock = 0°/25°
12 o'clock = 90°/25°
9 o'clock = 180°/25°
6 o'clock = 270°/25°
you can see that in this effect tilt doesn't change. only 1 parameter changes, so you're actually looking for a linear effect.

and just as a side note: 95% of all moving lights can't do this movement because they don't have infinite pan. usually you can do 1.5 turns (that's 540°).

this whole math changes (or actually the math is still the same but the results are different) as soon as you try to do a circle where 0°/0° is outside of the shape. so e.g. if you tilt a moving light to tilt 90°, effect 901 will give you an actual circle.

so how can i make the circle effect 

so how can i make the circle effect 

around the home position you can't make a circle effect because of physical limitations of the fixture.

You probably just need to set the pan and tilt of the fixture to somewhere in the middle of their range before applying the effect, maybe adjusting the scale to get smaller or larger movements as required.    Its likely its not working for you as you hit the maximum value of one or both of the pan and tilt.

The effect basically applies a sine wave/cosine wave pattern to the pan and tilt and because sin squared + cos squared = 1 that plots a constant radius if the light was pointing straight at a perpendicular surface when it started.

Obviously in the real world the geometry is not set up like that with the light proabbkly at 45 degrees to the surface and if the pan and tilt are not calibrated on the fixture profile properly (ie its not an etc profile but a custom one) then you are going to get an oval shape of some kind.  But the general look will be that the light is "spining" around.

A real circle effect is only possible with old style mirror head fixtures like the Clay Paky Golden Scan.
Or for use with S4 the Rosco I-Cue mirror extension.