Do not try and built a complex formula in one go.

Take it step by step; make LeaveType="Casual Leave" work first..

You have already written down the pseudo code in your description,
Now... best done in Notepad.. rewrite the description into a Formula..

Once you think it might work, paste it into SharePoint... If it fails.. go back to Notepad..

Tip: copy paste each try as a new paragraph in Notepad.. you learn from reviewing previous mistakes.

If Leave Type=Casual Leave 
   then I have to compare the field casual leave 
       in PTO Available 
   and I have to 
       subtract the calculated days of that particular column 
and the result should be 
    in Current PTO YTD 
    in Current Casual Leave PTO YTD .

Notes:

• Calculated Columns can not set a value in another Column, you need a Workflow for that

Do not try and built a complex formula in one go.

Take it step by step; make LeaveType="Casual Leave" work first..

You have already written down the pseudo code in your description,
Now... best done in Notepad.. rewrite the description into a Formula..

Once you think it might work, paste it into SharePoint... If it fails.. go back to Notepad..

Tip: copy paste each try as a new paragraph in Notepad.. you learn from reviewing previous mistakes.

If Leave Type=Casual Leave 
   then I have to compare the field casual leave 
       in PTO Available 
   and I have to 
       subtract the calculated days of that particular column 
and the result should be 
    in Current PTO YTD 
    in Current Casual Leave PTO YTD .

Notes:

• Calculated Columns can not set a value in another Column, you need a Workflow for that

    IF([Leave Type]="Casual Leave"
       ,"I have to compare the field casual leave"
       .....