I figured out the solution to this error message you might get with inputting the dumped file into pknx.
THIS IS A FIX FOR DUMPING FROM YUZU, ANYTHING ELSE I CANNOT HELP YOU WITH
What most of you probably did (like me) was you dumped the cartridge that was from a folder directory. Instead, you need to take the game's file (nsp, xci, etc.) and click on file on the top left, then press "Install file to NAND". Doing that, you then select the game's file and wait for it to finish loading.
I also CANNOT supply you with the update or game, you will ne