This would be a lot easier to debug, if you would provide your script.
The identifier "Inventory" isn't a valid type (not a script or class), or couldn't be found on base "self".
You're likely doing something along the lines of
var test : inventory - but as there is no built-in class with that name and you haven't provided a custom class either, it's considered an invalid type to define a variable with.
The class "Player" couldn't be fully loaded (script error or cyclic dependency)
You're likely doing something like this:
if get_parent() is SomethingElse:
if get_parent() is Player:
Each script depends on the other, so when loading one you enter an endless loop of dependencies (a so-called "cyclic dependency"). Also there is this issue.