Buying Seishun 18 Kippu pass online

Hi, I am travelling japan from 02/04/2020 to 11/02/2020. For my stay in that period, I want to use Seishun 18 Kippu pass but I am not able o buy it online. I saw the sale period id from 20 Feb to 1 March and validity is from March 1 to April 10. Can I buy the pass in Japan during my stay there or is there any way to buy it online.

I dont believe you can buy it online.

Can I buy the pass when I visit there 2/04/2020 to 11/04/2020

Are you going to be in Japan from April 2 – April 11?
Then, unfortunately, no. The sale period is from Feb. 20 to March 31, 2020 for use during March 1 – April 10.

(Please note that not everyone writes the dates as dd/mm/yyyy, which you seem to do, I am assuming…)

If I understand correctly, you type 11/02/2020 wrong and should type 11/04/2020 right, if I understand correctly. You might buy the ticket at unofficial discounted ticket shop. The price is slightly more expensive per day, but it only your choice. The ticket is second-handed and you can buy the ticket with 1,2,3,4 boxes available. I found this Japanese website with a lot of information (You might need to use
translation tool within your browser) https://seisyun.tabiris.com/kounyu.html

You don't need to buy it online. After coming Japan, you can buy this pass almost all JR station for 12050 yen,cover 5 days unlimited travel