Hello @Pasithee,
Welcome to OpenPetra!
I just tried on https://demo.openpetra.org, and I was able to create a PDF.
Please check the log file on your server, in directory log, the file Server.log
e.g. /home/op_dev/log/Server.log
There should be a error message.
You are welcome to post that error message here, and I can help you to solve it.
Please remove any sensitive data from the error message before you post it here in public.