Almost all my supports starts in mid air with default settings  

Ok lets get some basics out of the way. 
First of all you don't mention what version of Prusa Slicer you are using.  Or your OS.  Or much of anything really.
You haven't given anything to go on.  Normally you would need to provide at MINIMUM the model you are trying to slice.  However as settings play a HUGE part of the slicing result the way to provide everything is to add your model and slice to demonstrate the issue. 
Then to use FILE>Save Project As to save a Prusa Slicer *.3mf file.  The 3mf project file will contain your model(s), all your current settings, modifiers , orientations etc.  Its a snapshot of your system and is the only practical way to let others see what you are having trouble with.

Once you have a 3mf file (and this is the important part) you MUST zip it up into a zip file archive and attach it to a post here, use the choose file button on the bottom of the post field.  If you don't zip it then the forum wont accept the file.  You wont get a warning, it just wont attach.

Once we have a project file then you might get some sensible answers.

I will point out though that PS2.3.x and earlier have some rather sub optimal support handling unless work rounds and other tricks are used.  PS2.4 (currently in alpha) has many improvements to supports but for now just please post your project for people to look at.

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Sorry for the lack of detail. I use latest slicer with latest windows. I get this result every time when I try auto support. Ill try 2.4 alpha maybe helps

Exactly same in the 2.4 alpha 3 🙁 I have tried with several models

As mentioned, post your project or we cant help.