Sorry about the inappropriate error message; I'll fix that in the next build. The actual issue is that plink2 --score replaces the 'sum' modifier with direct control over the output columns. "plink2 --help score" has the following text toward the end:

  The main report supports the following column sets:
    maybefid: FID, if that column was in the input.
    fid: Force FID column to be written even when absent in the input.
    (IID are always present, and positioned here.)
    maybesid: SID, if that column was in the input.
    sid: Force SID column to be written even when absent in the input.
    pheno1: First active phenotype.
    phenos: All active phenotypes, if any.
    nmissallele: Number of nonmissing alleles.
    denom: Denominator of score average (equal to nmissallele value when
           'no-mean-imputation' specified).
    dosagesum: Sum of named allele dosages.
    scoreavgs: Score averages.
    scoresums: Score sums.
  The default is maybefid,maybesid,phenos,nmissallele,dosagesum,scoreavgs.

Note the "scoresums" option at the bottom. This means that if you add "cols=+scoresums" to your --score invocation, that will add the sum column you're looking for.