Website builders in WordPress tend to be big and sow and need big hosting to run properly. Free hosting is free, so the amount of server power you get is limited.
That said, that’s mostly an educated guess. Maybe we can find out more about why it breaks, and get a real error message from PHP.
For the generic HTTP ERROR 500, you can enable display errors like this:
But WordPress overrides this setting and has it’s own debug setting instead:
If you enable these options and then try again, you should see a more technical error message that may give more information.