Your domain matches a list of high-profile phishing targets, most likely due to it’s similarity to rbc.com (Royal Bank of Canada). CAs are obliged to maintain such a list and use additional verification mechanisms when verifying domain ownership for such domains. That usually comes down to manual verification, which wouldn’t work in the case of Let’s Encrypt due to its automated nature, so domains on this list are essentially prohibited from requesting certificates.
The list does occasionally get amended based on user feedback, but I’m not sure if that would be appropriate here.