I tried to use this add-on. I can dowload and install it as described in documentation.
When I enable it in Project Settings, Godot returns the following issues:
res://addons/godot_db_manager/db_interface.gd:15 - Parse Error: The identifier "gddb_constants" isn't declared in the current scope.
modules/gdscript/gdscript.cpp:599 - Method failed. Returning: ERR_PARSE_ERROR
res://addons/godot_db_manager/dlgs/new_db_dlg.gd:30 - Parse Error: The identifier "gddb_globals" isn't declared in the current scope.
modules/gdscript/gdscript.cpp:599 - Method failed. Returning: ERR_PARSE_ERROR
However, I can start it in Project>Tools and the main window appears. After this point, the commands in File>New DB/Load DB don't answer.
OS : Windows10 Godot 3.2.2.stable
The error occured for downloads from the Godot Engine and Github.