If this is your goal, why are you specifically trying to install the DNS plugin?
Most of the time, people need the DNS plugins for wildcard support. Also, "creating" a subdomain without any relation to the TLS certificate, isn't related to certbot or Let's Encrypt.
Not necessarily. The dns-01 challenge for which a DNS certbot plugin is very helpful, is only really necessary if you want or need a wildcard certificate or if port 80 or 443 isn't accessible.
If you've managed to get a certificate for your hostname succesful previously and just need one or a few subdomains (i.e.: don't need a wildcard certificate) then you don't need the dns-01 challenge and subsequently don't need a DNS plugin.
You can just add the subdomain to the certbot command you used previously for your hostname by adding another -d option with the subdomain after it.
I’m not sure I understand. Just add more -d <hostname> options to the previously used certbot command? Let’s Encrypt certificates can contain up to 100 hostnames in one certificate.