Are you aware that you cannot create a disposable template from a template?
Your disposable template needs to be created from an App VM. I suggest you duplicate an App VM that’s based on your preferred template and go from there.
Then go to the Advanced tab in the Settings for your duplicate, and you will find the ‘Disposable template’ is not greyed out, while in the Advanced tab of Settings in any regular template, the ‘Disposable template’ is greyed out.
Does that help?