Previous post / next post

Hi
I recently opened a blog https://www.news4kid.com and everything seems to work fine but i have this problem . On my main page i was able to add next page/ previous page on the main page as i had more than the 12 posts allowed on any page . It works perfectly for me .

However i have not been successful in doing the same in the category page. For eg i have appx 21 posts in the science category but i cannot go to next page in the category page . How do i do that?

I tried to download the Next Page plugin 1.5.2 but it does not work . What am i doing wrong and how can i improve it

The blog I need help with is news4kid.com.