@sudhanshubhasin For your Win7 / Chrome client, you might try these two things from this earlier thread.
Per @webprofusion
Browse to this site using Google Chrome: https://valid-isrgrootx1.letsencrypt.org/ in chrome, then go back to your site and see if it works. (Note he suggested the Win7 machine might be missing ISRG Root X1 in your thread earlier).
If that does not work, try these instructions @mel_mel posted. It is more complicated and read through the posts following it for full explanation. It sounds like you can not, and probably should not, load the intermediate cert but follow the rest of instructions.