Watalii wanahitaji kwenda ofisi ya Uhamiaji. Cannot report this, since it is only a 'spelling mistake'.
why isn't it ofisini?
Wouldn't it be "ofisini", to indicate that is where they are going?
Why is ofisi plural here?
ofisi ya uhamiaji = immigration office / ofisi za uhamiaji = immigration offices. See my first comment in this thread.