Please fill out the fields below so we can help you better. Note: you must provide your domain name to get help. Domain names for issued certificates are all made public in Certificate Transparency logs (e.g. crt.sh | example.com), so withholding your domain name here does not increase secrecy, but only makes it harder for us to provide help.
My domain is: www.fishcc.org
I ran this command: certbot certonly --webroot -v
It produced this output:
Saving debug log to /var/log/letsencrypt/letsencrypt.log
Plugins selected: Authenticator webroot, Installer None
Please enter the domain name(s) you would like on your certificate (comma and/or
space separated) (Enter 'c' to cancel): www.fishcc.org
Requesting a certificate for www.fishcc.org
Performing the following challenges:
http-01 challenge for www.fishcc.org
Input the webroot for www.fishcc.org: (Enter 'c' to cancel): /root/apache-tomcat-9.0.82/webapps/ROOT
Waiting for verification...
Challenge failed for domain www.fishcc.org
http-01 challenge for www.fishcc.org
Certbot failed to authenticate some domains (authenticator: webroot). The Certificate Authority reported these problems:
** Domain: www.fishcc.org**
** Type: dns**
** Detail: no valid A records found for www.fishcc.org; no valid AAAA records found for www.fishcc.org**
Hint: The Certificate Authority failed to download the temporary challenge files created by Certbot. Ensure that the listed domains serve their content from the provided --webroot-path/-w and that files created there can be downloaded from the internet.
Cleaning up challenges
Some challenges have failed.
Ask for help or search for solutions at https://community.letsencrypt.org. See the logfile /var/log/letsencrypt/letsencrypt.log or re-run Certbot with -v for more details.
My web server is (include version):apache-tomcat-9.0.82
The operating system my web server runs on is (include version):Ubuntu16.04.12 64bit
My hosting provider, if applicable, is:https://www.aliyun.com/
I can login to a root shell on my machine (yes or no, or I don't know):yes
I'm using a control panel to manage my site (no, or provide the name and version of the control panel):no
The version of my client is (e.g. output of certbot --version or certbot-auto --version if you're using Certbot):certbot 2.7.4
DNS record:
testFile:http://fishcc.org/.well-known/acme-challenge/testFile
letsencrypt.log: http://fishcc.org/.well-known/acme-challenge/letsencrypt.log
I guess my DNS record is correct, IP address is public IP and test file can download from my web site successfully, but it still didn't work. 
I don't have any idea about how to do next, someone can give some help? Thanks. 
It works. I have already added the www subdomain to my DNS records, thanks a lot.