I having issue that content assistant / intellisense is working in methods such as onCreate but when I am calling it inside onClick method its not working.
I have attached screen shots of both scenarios.

Above image show its working when I call it using ctrl + space in oncreate method

Above image its not working
Also I am getting this error “The method onCreate(Bundle) of type ACTIVITY NAME must override or implement a supertype method”
shown in image

But activity is already overriding super method.
This is only happening in this activity, other activities content assistant is working.
Have you tried to Reset To Default Option in :
Windows -> Preferences -> Java -> Editor -> Content Assist -> AdvancedIn that “Api Proposal tools” is Main.
Then it will work, in that above window all options are for intellisense ( Ctrl + Space) only, just have a look, if gets confusing then just set it to Default and ya that’s it.