Not able to sell the aircraft

Hello folks,

I am planning to sell the aircraft I purchased, but could not do it. I see all the options from 10%-90%, but would not let me select anything. 

Do you all know what's the reason behind it?

I guess the yellow text in your picture discribes the problem. You can see your equity when you click on your money --> balance sheet --> on the right bottom side

You will find the reason why at the bottom of your screenshot (in orange letters) ... to act an aircraft lessor ......

See also http://community.airlinesim.aero/topic/11125-leasing-out-aircraft/