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Your camera's battery is actually contained within the film cartridge. That is why it is a bit larger and heavier. The way to replace the battery is to replace the print cartridge. Being that the flash doesn't work, that means that the film/battery cartridge must be replaced.
This is explained on the following page: https://www.techwalla.com/articles/how-to-change-batteries-on-the-polaroid-600-onestep-express
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