Vi - Hidden missing tab in Makefile

Makefile:<line_num>: *** missing separator.  Stop.

Most probably because of missing TAB in the begining of the line. you know this, you verified that you are starting line by pressing TAB in your Vi, and still you are getting this error. Why?

Try
:set et?

and if it shows:
expandtab

then this is an answer. Expandtab is transfering your tabs into spaces (and this is very often demanded Vi setup by project coding standard).

Solution:
In instert mode press CTRL+v, <tab>
This will insert real tab char.

Verify that line begins with real <TAB> char by issuing Vi command :
:set list