I use IntelliJ IDEA v2021.1.3 and as mentioned on this page, I checked on View | Tool Windows | Bookmarks
to view all bookmarks, but there is not any menu or opened window available as shown on the screenshot. So, is there any bug? Or how can I open bookmarks window toolbar?
The Bookmarks feature is new in IntelliJ 2021.3. See What’s New page.
You have to update to version 2021.3 to have that feature. If you see the page you indicated, the same is for 2021.3 version.
Your version does not have that link.
In 2021.3:
See the images bellow from https://www.jetbrains.com/help/idea/bookmarks.html#add-bookmarks and https://www.jetbrains.com/help/idea/2021.1/discover-intellij-idea.html.