The type or namespace name 'Foo' could not be found (are you missing a using directive or an assembly reference?)
Save, and check if there are compiler errors in Unity, if there are no errors listed, then the issue isn't a real issue and your IDE needs to be refreshed.
Make sure you follow the autocomplete suggestions of your IDE and do not type things manually.
If your IDE isn't showing errors, you will need to configure your IDE.
If the type is nested in another class you must specify the outer class to access the inner.
OuterClass.InnerClass
for example.
The type may have changed name, or doesn't exist in the version of Unity or the package you are trying to reference. Check the documentation, ensuring you are looking at the correct version.
Sorry, we've run out of troubleshooting! If you resolved your issue and the fix was not listed in the troubleshooting steps, please report an issue with this page .