My well is set to be not in operation, but I want to toggle this on, but the slider for “Is in operation” is now greyed out. Why?

If a well exists and you have run at least one scenario with this setting toggled off, pForecast will assume that your well is in production and prevent you from changing it by accident. To alter this setting, you will need to delete your previous scenario, then toggle this slider on, to be able to provide a different schedule for this well.