You have an IDE issue#

Save, and check if there are compiler errors in Unity's Console window (WindowGeneralConsole, CtrlShiftC).
If there are no errors listed, then the issue isn't real, and your IDE needs to be refreshed.

1. If it's present, select regenerate project files in EditPreferencesExternal Tools.
2. Restart your IDE.

You have misspelt the type or namespace#

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.

Your code is incomplete and the type was not provided#

//  Incorrect. A variable requires both a type and a name.
public void Example(myVariable)
{
    ...
}

If your IDE isn't showing errors, you will need to configure your IDE.

You are missing the full path to the type#

If the type is nested in another class you must specify the outer class to access the inner. OuterClass.InnerClass for example.

The type doesn't exist#

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.