Hi @AngelicaGandica,
You have two problems here, the first one, as @schoen already said, you have no A record defined for domain dropbite.com but you have a second problem for www.dropbite.com, Let’s Encrypt can’t reach the challenge.
For the second problem, to be sure you are using the right Document Root for your domain and that the challenge file could be reached, create a test file and try to get it.
mkdir -p /opt/mattermost/.well-known/acme-challenge/
echo -n "Testing acme-challenge" > /opt/mattermost/.well-known/acme-challenge/test
Now use your browser to reach the test file http://www.dropbite.com/.well-known/acme-challenge/test
Or using command line:
curl -ikL http://www.dropbite.com/.well-known/acme-challenge/test
If you get the text “Testing acme-challenge” then it is ok, if you get another thing then… or the document root is not the right one or maybe your application Mattermost is doing something strange with these files.
If the problem is Mattermost application you could configure nginx to use a workaround but first, try the test.
Cheers,
sahsanu