Student or Learner
1) We took camera with us but didn't use it in the end.
2) I rushed to the station but the train was late and in the end I had to wait for 20 minutes.
So my question is, can we use "At the end" here. I mean why we didn't use "At the end here"?
In those sentences, only "in the end" is correct. "At the end" refers to location (geographical or in some other way physical.)
At the end of the story ...
At the end of the railway line ...
At the end of the 19th century ...
"In the end" means "The eventual consequence".
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.