Used Metal Knife on Aluminum Cake Pan, is it Ruined?
Steel (as used in knives) will always scratch aluminium, which is really quite a soft metal. Glass and even some plastics can mark it quite noticeably as can storing aluminium pans stacked together. In other words it happens. Don't worry about it. The pan doesn't look as nice but it's not ruined. The main reason to avoid scratching it is to avoid getting fragments in the food.

I have a set of miniature pudding basins made of aluminium and if things don't just fall out, the only tool I can find that helps is a butter knife (they're too curved for any of my silicone tools or plastic pallette knife to get in nicely). But an old, smooth, worn one, and that doesn't scrape material off (though it does leave a mark.)