以下の様にnginxでproxyを構成している前提

    _____         +-----+             +------+
 __(     )_       |nginx|             | WEB  |
(_Internet_)======|proxy|=============|Server|
  (______)        +-----+             +------+

acme.shはこの前段のnginxで動作しているものとします

$ acme.sh --set-default-ca --server letsencrypt_test
 Changed default CA to: https://acme-staging-v02.api.letsencrypt.org/directory

# acme.sh --register-account -m yourname@example.com
[...... 2022] Registering account: https://api.buypass.com/acme/directory                                　  
[...... 2022] Registered                                                                               　 
[...... 2022] ACCOUNT_THUMBPRINT='-DxxxxxxxAAAAAAAABBBBBBBBBBBBBBbCCCCCCCC'   

mkdir -p /usr/local/www/acme_root/.well-known/acme-challenge
chown -R www:www /usr/local/www/acme-root

これは別ファイルとして必要に応じて組み込む

/usr/local/etc/nginx/includes/acme-webroot

location /.well-known/acme-challenge/ {
    alias /usr/local/www/acme_root/.well-known/acme-challenge/;
}

/usr/local/etc/nginx/sites-enabled/default

server {
    listen 80;

    server_name mydomain.com;

    # ....

    # Let's Encrypt webroot
    include includes/acme-webroot;
}