Hello, thank you for getting in touch and for your support!
Setting the 'Variable Mode' to 'Fetch' won't declare it over the shader, meaning that it has to be declared over an included cginc, for example, so the error is being thrown since the shader can't find any reference to that property name.
If you want the property to be created on the shader, you need to set this parameter to 'Create'.
Please let me know if this helps and if you have any further questions.