That's an error message from GNU Make. I'm not sure what version of GNU Make you're using, because GNU Make has emitted the following since 1998:
Makefile:2: *** missing separator (did you mean TAB instead of 8 spaces?). Stop.
See 2c64fb221a265f9e7fc93374906b1e7540377561:
1998-09-04 Paul D. Smith
* read.c (read_makefile): If we hit the "missing separator" error,
check for the common case of 8 spaces instead of a TAB and give an
extra comment to help people out.
I guess you may have indented it in the modern way with 4 spaces of "tab". Either way, there's also help to be had from your other tools, e.g., vim highlights space-indented make recipes in glaring red error bars.