The error 'Unity does not contain a definition for' generally occurs when you're trying to access a class, method, or property that doesn't exist in your current context. This could be due to several reasons:
Typographical Errors: You might have misspelled the name of the class, method, or property.
Scope Issues: The class, method, or property might not be accessible from where you're trying to use it, possibly because it's private or protected and you're trying to access it from outside its class or assembly.
Incorrect namespaces: If you did not include the correct namespace for the class, or if the class is inside a different namespace than you are currently working in, then Unity will not be able to find the class and will throw this error.
Non-existent elements: You're trying to call a class, method, or property that doesn't exist at all. This can occur if you expect a library or package to include certain functionality which it doesn't.
To resolve this error, consider the following solutions based on the causes discussed above:
Check for Typographical Errors: Ensure that the name of the class, method, or property is correctly spelled and matches exactly with its declaration including case sensitivity.
Address Scope Issues: Make sure the class, method, or property is publicly accessible or you are accessing it within its scope. If it's declared as private or protected and you need to access it, you might need to make it public, or create public methods to interact with it (following good encapsulation principles).
Verify the Correct Namespace: Make certain that you've included correct namespaces in your script. If the class is a part of a different namespace, include it at the top of your script with using directive.
Check if Elements Exist: Confirm that the class, method or property actually exists in your libraries or packages. If not, you might need to download and install additional libraries or adjust your code to work with the existing ones.
By addressing these issues, you should be able to resolve the 'Unity does not contain a definition for' error.