Login keys functionality is turned off in DirectAdmin. It has to be enabled by modifying a file as shown in plugin installation guide. If the problem persists after enabling login keys try to re-install the plugin.
Please try to create login key manually:
1. go to /CMD_LOGIN_KEYS in your panel (URL)
2. click on "Create new Login Key"
3. fill in key name: "siteprobuilder", key value: [think of some 16-char code], expires: never, uses: 0, clear key: checked, allow htm: checked, commands: CMD_LOGIN and CMD_PLUGINS.
4. enter password below and click "Create".
If this operation succeeds then create a new file if it doesn't exist already:
/usr/local/directadmin/plugins/siteprobuilder/include/login_key and put there a key value you used in login key creation. Save the file.
After that if everything is succeeded then publishing to this server should start working.
Make sure that IP of used builder server is not blacklisted in DirectAdmin. Make sure that IP is not listed in list of blocked IPs in file:
/usr/local/directadmin/scripts/custom/all_pre.sh (if exists).
PHP 5.6 or newer require a valid SSL certificate to connect. So the API call will fail if DA SSL cert is invalid. You can use PHP 5.5 to fix this problem.