Hello, unfortunately it's a limitation with how the supercard works - we're not able to change the game to work with its ROM patching as it relies on patching over Nintendo's SDK code (which we did not use).
It should still be possible to save with the supercard, but it sounds slightly fiddly: After playing, quickly turn the GBA off and on again, restart the menu, go to the saver tab and copy the SRAM to the SD card. (I believe this solution may rely on the internal battery in the supercard still holding charge but the documentation is a little unclear).
Hope that works ok!