Hi!
The DSi is still able to read the SD cards (as it’s saying that it cant be used). This is most of the time caused by the SD card it self. How is the SD card formatted? Be sure it’s FAT32 formatted.
The official max size supported is up to 32GB (as those cards can be formatted in FAT32).
More info here: https://www.nintendo.com/en-gb/Support/Nintendo-DSi-XL-/Basic-setup-and-use/SD-Cards/Compatible-SD-Card-types/Compatible-SD-Card-types-245271.html#:~:text=The%20Nintendo%20DSi%20is%20compatible,over%202%20GB%20in%20size.
Please let us know how it goes!