The error message wasn’t specific enough in this case. We had a rate limit of sending only resending 30 activation emails per hour by IP address.
I’ve fixed the error message and also removed that rate limit for logged in staff users doing it via the admin interface.
I am curious @Mischief, why are you resending so many activations?
https://github.com/discourse/discourse/commit/46ca66771be750305a3437f09773465c6f6a649b