As the user Leandro wrote on facebook wrote: Bricks Community | Facebook
The Sandbox seems to be broken. I tried it for myself and if you try to add a template and edit it or to edit a page you get the message “you’re not allowed to access this page”.
I’m getting the same issue, I was interested in trying the sandbox before committing to any of the plans. I can’t find any official documentation on the sandbox, I just happen to come across it within a YouTube video, in terms of features I couldn’t work out if it was limited or if there were issues as you’ve mentioned.
Thanks so much for the heads-up. Should be working now. Happy testing 