This extensions writes the data of the guestbook in a separate file (i.e. guestbook.html). Because the Apache PHP module writes the data, you have to take care about the file permissions on the server so that PHP can access, open and write the file.

In order to adjust the file permissions, transfer the local guestbook.html file to the web server. Then select the file on the server side and you will see an inspector somehow similar to the above screen.

Make sure that apart from you as the file 'owner' at least the 'group' of the owner has write permissions too. If that does not work, give 'others' write permissions too - and don't forget to set the right with the button and test it in your browser with a fresh, cache clean setup.