Akash Agrawal I was validating in validator.w3.org site using Sublime text editor in my pc. Now it is solved. Code were working , but there were some tags error.
Sololearn playground helped me. Thanks for your guidance.
If you wanted a list inside a list,
You missed a closing li tag after the nested ul element.
<!-- You missed the below closing li tag -->
Check and correct your code. It should work