I recently started reading Sergeant Keroro manga, but few volumes from 17 to 20 are not available. Is there any other site where I can read the missing volumes?
according to Manga Updates there are 27 volumes in Japan.
however. while licensed in English, Manga Updates lists that Viz Media has dropped the series at Volume 20 but Kadokawa ComicWalker is listed as Ongoing but is only on Volume 17 but the website lists the ebooks up to volume 20.
while the link to Stg. Frog on ComicWalker has a Read Latest button this only shows 8 pages and i don't know what volume it's from however i would suspect this is Volume 20
So at the date this answer was posted (21/10/2017) there is no localized copy of Volumes 21-27.
it should be noted however that ComicWalker and BookWalker (which ComicWalker links to) are English services of Kadokawa Shoten who are the ones who publish Sgt. Frog in Japan as such they would be your best place to find out where and when new volumes come out.
as for reading them online if future volumes come out, both sites seem to only have a preview button while at the bottom of a product page on BookWalker it says
Please check your device (iOS, Android) supports the BookWalker app before purchasing by downloading the app when you will use the app.
as such i don't think there is any website where you can read Sgt. Frog legally