If DBI or Goldleaf successfully installs the NSP as a base game, you have confirmed that the file is indeed a base game mislabeled as an update.
Preventive practices
nscb builder -i unpacked_folder -t nsp -o fixed_update.nsp the nsp file is missing a programtype nca upd
If you're still experiencing issues, providing more details like the specific tool you're using, the source of the NSP file, and any steps you've taken can help in narrowing down a more targeted solution. If DBI or Goldleaf successfully installs the NSP