This is the most critical component. It contains the actual game binaries (ExeFS) and the game data (RomFS). Without this, the system has no executable code to run.
If the file size seems correct, the archive itself is likely "dirty" or corrupted. Re-dump the game from your physical cartridge or re-acquire the file from a verified source. Check Your Signature Patches (Sigpatches): the nsp file is missing a program-type nca