Hint: The Certificate Authority failed to download the challenge files from the temporary standalone webserver started by Certbot on port 80. Ensure that the listed domains point to this machine and that it can accept inbound connections from the internet.
can someone help me with this. i have check all ports and i can see anything blocking the port 80
Welcome to the Let's Encrypt Community! 
Without knowing your domain name it will be nearly impossible for us to help you with this.
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mail.patnuc.cm
thats my domain name
Hello sir
mail.patnuc.cm
The --standalone method is difficult to debug because you need to keep Certbot running to test connection from the public internet.
Probably the easiest way to test is with these command options:
certbot certonly --standalone --dry-run --debug-challenges -v -d (domain)
This command will show you the challenge URL to try from the public internet and the proper response. After showing you this it will say "Press Enter to Continue". DO NOT PRESS ENTER.
Leave it paused and use a different device to test connection. You can use a mobile phone with wifi disabled to use your carrier's network.
You do not have to use the full URL. Just try http://(domain)
If the connection works this shorter URL should see a response like below. I am pretty sure you will get a timeout error instead just like Let's Encrypt did. Repeat this as needed as you modify your comms setup until it works.
ACME client standalone challenge solver
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alright