I just created a @Stereotype and the container claimed that:

weld-001317 interface ... is not a stereotype

Luckily I have the spec opened, so I identified the problem fairly quickly:

A bean stereotype is a Java annotation defined as @Target({TYPE, METHOD, FIELD}), @Target(TYPE), @Target(METHOD), @Target(FIELD) or @Target({METHOD, FIELD}) and @Retention(RUNTIME).

I had omitted the @Target, and when I set it, the problem was fixed.

So, my suggestion is - in case an annotation annotated with @Stereotype does not define @Target, Weld should be more informative and state the exact problem, rather than leaving the person wondering.