So I just started learning any% shovel knight. I got to the Flying Machine level, and was just by the item that you need to buy, but I only had 2,000 gold when I needed 4,000. It might have been because I died a lot, which hopefully won't happen later because I was kind of messing around. It's not a big deal now because I wasn't in a run, but was if this happens when I am in a run? Is there a backup or do just I need to reset?
Ideally if you're doing pretty well in runs, getting to the point where your PB is sub hour, this would be a reset for you since it'd be pretty rare.
If for some reason you were keen on not resetting, the gem stage that appears on King Knight's stage later in the game offers plenty of cash, and if you want something else quick you can play Plains again--but this would lose a lot of time. If you happened across a music sheet you could sell them before entering the stage as well.
Your other option is to try to play it safe and grab more gold earlier. Breaking easy checkpoints here and there is good, especially if you would otherwise skip it. You may want to consider opening some secret walls with chests or gold while doing your run, to not go too far out of your way and grab some extra money.
It's important to have a gold "route" where you keep rough track of how much money you should have at each stage of the game, and how much is required for major purposes.
Every time you die you should make sure you get your money back if possible if you haven't bought all of your relics yet. You never want to be in a situation where you're unable to get the relic needed to finish the run. You can try to break some checkpoints if you're desperate.
