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.
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.