Hi, Filipe.
The UserId parameter can be set using the GetUserid() function.
If that is what you are using, make sure that your database rules, in Firebase Console are set.
You can set them by following the image below.

These are insecure rules. You can learn more about these, by following this link.
Furthermore, if you need a more in-depth guidance on how to set it up, here's a useful link.
Hope this helps.
Cheers,
Rui Mendes