Hi @Bishwas. Unfortunately it is no longer possible to subscribe to Spotify through iTunes.
Up until recently, it was possible to pay for Spotify Premium using Apple’s in-app payment system (iAP). However, this has been discontinued for new subscribers. This is because Apple required an additional charge on top of the standard rate, so we decided to cut out the middleman to make things cheaper for you! -Source
This page will provide you with a list of payment methods available to you. Hopefully there will be one that will work for you. Please let me know if you have any other questions or concerns I might be able to assist you with. Best of luck!