pgfplotstable: Highlight rows of a numeric sequence

handlers/.list uses foreach to unfold the list:

foreach pgf@keys@key in{#1}%

while pgffor handle the list in different ways by checking if there is a leading bgroup:

defpgffor@@vars@end in{%
  pgfutil@ifnextcharbgroup{pgffor@normal@list}{pgffor@macro@list}%
}

As you see macro list (unbraced) is expanded to a normal list (braced) first:

defpgffor@macro@list#1{%
  expandafterpgffor@normal@listexpandafter{#1}}

So when you write highlightrow=List and expect it to be processed as a macro list, but actually List is braced and be processed as a normal list and then error occurs.

The fast way to fix the problem is what Ulrike Fischer has mentioned in her comment: use highlightrow/.list/.expand once=List.

I provide another way to make it possible to use a function to get the row index with expl3.

documentclass[margin=1cm]{standalone}
usepackage{pgfplotstable}
pgfplotsset{compat=1.17}
usepackage{xparse}
pgfplotstableset{
  highlightrow/.style 2 args={
    postproc cell content/.append code={
      ifnumpgfplotstablerow=#1
        pgfkeysalso{@cell content={#2##1}}%
      fi
    },
  },
}

ExplSyntaxOn
NewDocumentCommand { rowstyle } { O{30} O{#1} m }
  {
    row_style:nnn { #1 } { #2 } { #3 }
  }
int_new:N l__i_int
int_new:N l__index_int
cs_new_protected:Nn row_style:nnn
  {
    cs_set:Nn __parse_expr:n
      {
        fp_to_int:n { floor(#2) }
      }
    fp_compare:nT { #1 >= 0 }
      {
        int_zero:N l__i_int
        fp_while_do:nn { __parse_expr:n { l__i_int } <= #1 }
          {
            tl_set:Nx l_tempa_tl {
              exp_not:N pgfplotstableset{
                highlightrow={__parse_expr:n { l__i_int }}{ exp_not:n { #3 } }
              }
            }
            tl_use:N l_tempa_tl
            int_incr:N l__i_int
          }
      }
  }
ExplSyntaxOff

begin{document}
rowstyle[11][#1*(#1 + 3)/2]{color{red}}
rowstyle[11][(#1 + 1)*4]{color{blue}bfseries}
pgfplotstabletypeset[string type]{
  A  B C
  0  x x
  1  x x
  2  x x
  3  x x
  4  x x
  5  x x
  6  x x
  7  x x
  8  x x
  9  x x
  10 x x
  11 x x
}
end{document}