By definition in HtDP (for which *SL exist), a template is an outline of a function (an â€śinventory of what we know about the function based on its purpose statement and the data def. for its input(s)â€ť). Hence â€¦ is allowed to show up inside of a function definition but nowhere else.

Concretely,

(define (f x)

are the first few keystrokes for entering a function definition. Check.

Similarly,

(define f (lambda (x)

could be completed to a function definition.

But,

(define x â€¦)

is not the beginning of a function definition. In the spirit of â€śsignal errors earlyâ€ť the *SL evaluators signal an error.

A reader is expected to know HtDP (somewhat) where this terminology is defined.
But adding â€śfunctionâ€ť to these explanations makes them somewhat more self-contained
so I pushed a fix to the docs (commit 9379cf6bdb083e8a32889504e1531addc890724).