t() strings can't able to translate
I have found the solution.

Initially, my site is in English(source language).

We have to switch the language to other languages on the site(in my case it is Dutch) using this path admin/config/regional/language. Then, we have to view the page which is consists of t(). Then only the t() strings would register for translation.

Now go to admin/config/regional/translate/translate and search your t() string, it will be available and add translation !!!