Hi Nawras vray lens effect doesn't work on my vray 3.6 why ?

Short answer :
Don't lower burn value under color mapping and reduce it from th VFB when needed.
Long answer :
In the recent vray updates (3.5+ iirc) they've changed the lens effect behavior in that regards.
The new workflow except the render to be in linear meaning no highlights compression nor any color mapping applied to the exposure range or the lenseffect won't work as expected.
The new workflow suggest using the color mapping defaults ( burnvalue 1.0) and change the burn amount from the frame buffer directly if it's necessary.
Tip 1:
Following this workflow will make the "contrast" control in the frame buffer work in much more efficient and predictable way so I would highly recommend it even if you don't need to use any lens effect in your scene.
Tip 02 :
Because of the nature of how Vray image sampler work you might find rendering the image in linear be a bit slower as the burned area could require more samples to reach the noise threshold compared to the amount of samples that's needed in case of lowering burn value and that's expected ( check image sampler section in Vray official document for further information)
The workaround for that is to ask the renderer to do the sampling using low burn value And in the viewed render to show the untouched linear image so you get the best of 2 worlds (speed and correct VFB behavior for lens effect and color correction controls), to do thst just use the desired burn value (usually I use as low as 0.05 but you can test different values) and under color mapping mode chose "None".