I have a Firebase project with 2 mobile apps:
The Firebase Database is a key-value store. It stores the values of keys at paths in the tree. But it only stores keys when they have an actual value. If there is no value for a key/path, it is immediately deleted.
So if you click the + button, then enter a key and click the Add button, it will delete the first key that you entered because it doesn't have a value.
What you instead should do depends on what you want to accomplish.
To add a single key/value pair:
If you want to add a JSON snippet/tree: