I have a product in my cart that is free, but because I do not have any current funds, it will not let me get it even tho it is free. Why is this?

Perhaps it's a DLC and you don't have the base game for it?

What game is this? Could you share screenshots of the game page and your cart?

If this is about "Enderal: Forgotten Stories", there is a bug or feature that lists it as a free game, but you need to own something else before you can claim it, so it's more like a free DLC.

I have made that mistake a couple of times, I have chosen the "free" game and during the checkout discovered that you need to buy something else to get it for free.

But as far as I'm aware of, that's the only case where that happens. Otherwise free should be free, and you don't need to enter any payment information to get those.

Are you sure it's because of lacking funds?

Because I would assume the same as PixelBoy:
you tried to claim the Skyrim Total Conversion "Enderal: Forgotten Stories"?

The hint that you need to own a legal copy of "Skyrim" (here on GOG) is a little more hidden, than usual:

"Enderal is free and can be played by anyone owning a legal copy of TES V: Skyrim."
(on the total conversion's page, under the picture of the map)

Usually, there's a notice saying something like: "To play this game, you also need: [name of main game]"

I guess, GOG didn't add that little info to this game, since it's not a DLC, but a total conversion? shrug

All freedom is conditional.