Welcome @calebwoodbine
I don't know the answer to your specific subdomain question but the error you show relates to issuing 5 identical certs in the past 7 days. This should not occur in a well-functioning system as certs are typically renewed once every 60 days.
This rate limit protects the Let's Encrypt system from poorly designed or run-away systems. There is no increase for this rate limit as it should not happen.
If testing, you should use the staging system.