When fermenting sauerkraut it's best to make the sauerkraut with fresh non frozen cabbage for best results.
But if you want to make sauerkraut with frozen cabbage you can do so but it will not turn out as good and will likely take a bit longer to ferment the sauerkraut when using frozen cabbage.
I prefer to use fresh cabbage that isn't frozen when making homemade sauerkraut.
The sauerkraut will ferment at around 70 F degrees so the cabbage will eventually thaw out during the fermenting process but it will also make the sauerkraut a bit soggy.
So that's another reason I would not recommend using frozen cabbage.