Как получить доступ к двойной карте с помощью Sass?

У вас есть карта с вложенными списки, а не карты.

Таким образом, вы можете создать цикл для этих списков с @each получением индекса в каждом цикле:

@each $item in $nth-key {
    $index: index($nth-key, $item); // => get the item's number at every loop
    &:nth-of-type(#{$index}):before { content: $item;}
}

Это полный код

@mixin generate-table-nth($page-name) {
    @if $page-name == 'indicators-page' {

      $table-list: (
        table-contract-details:       ('SHOPPING:', 'NOME FANTASIA (ATUAL):', 'CÓD. CONTRATO:', 'DATA ASSINATURA CONTRATO:', 'DATA INAUGURAÇÃO:', 'DATA INÍCIO CONTRATO:', 'DATA FIM CONTRATO:', 'PRAZO CONTRATO:', 'ATIVIDADE:'),
        table-contract-rent:          ('SHOPPING:', 'CÓD. CONTRATO:', 'NOME FANTASIA (ATUAL):', 'DATA INÍCIO ALUG. CONTRATUAL:', 'DATA FIM ALUG. CONTRATUAL:', 'ALUGUEL CONTRATUAL:', 'DATA RENEGOCIAÇÃO:'),
        table-percent-rent:           ('SHOPPING:', 'CÓD. CONTRATO:', 'NOME FANTASIA (ATUAL):', 'TIPO PRODUTO:', 'DATA INÍCIO ALUGUEL %:', 'DATA FIM ALUGUEL %:', 'VOLUME VENDA:', '% VENDA:', 'OBSERVAÇÃO:'),
        table-minimum-rent-reduction: ('SHOPPING:', 'CÓD. CONTRATO:', 'NOME FANTASIA (ATUAL):', 'DATA INÍCIO:', 'DATA FIM:', 'VALOR:', 'TIPO REDUÇÃO:')
      );

      @each $table-key, $nth-key in $table-list {
        $table-name: $table-key;
        &[data-element-name = "#{$table-name}"]{
          tbody {
            tr {
              td {
                @each $item in $nth-key {
                   $index: index($nth-key, $item);
                   &:nth-of-type(#{$index}):before { content: $item;}
                }
              }
            }
          }
        }
      }
    }
}

Ваш вывод:

table[data-element-name = "table-contract-details"] tbody tr td:nth-of-type(1):before {
  content: "SHOPPING:";
}
table[data-element-name = "table-contract-details"] tbody tr td:nth-of-type(2):before {
  content: "NOME FANTASIA (ATUAL):";
}
table[data-element-name = "table-contract-details"] tbody tr td:nth-of-type(3):before {
  content: "CÓD. CONTRATO:";
}
table[data-element-name = "table-contract-details"] tbody tr td:nth-of-type(4):before {
  content: "DATA ASSINATURA CONTRATO:";
}
table[data-element-name = "table-contract-details"] tbody tr td:nth-of-type(5):before {
  content: "DATA INAUGURAÇÃO:";
}
table[data-element-name = "table-contract-details"] tbody tr td:nth-of-type(6):before {
  content: "DATA INÍCIO CONTRATO:";
}
table[data-element-name = "table-contract-details"] tbody tr td:nth-of-type(7):before {
  content: "DATA FIM CONTRATO:";
}
table[data-element-name = "table-contract-details"] tbody tr td:nth-of-type(8):before {
  content: "PRAZO CONTRATO:";
}
table[data-element-name = "table-contract-details"] tbody tr td:nth-of-type(9):before {
  content: "ATIVIDADE:";
}
table[data-element-name = "table-contract-rent"] tbody tr td:nth-of-type(1):before {
  content: "SHOPPING:";
}
table[data-element-name = "table-contract-rent"] tbody tr td:nth-of-type(2):before {
  content: "CÓD. CONTRATO:";
}
table[data-element-name = "table-contract-rent"] tbody tr td:nth-of-type(3):before {
  content: "NOME FANTASIA (ATUAL):";
}
table[data-element-name = "table-contract-rent"] tbody tr td:nth-of-type(4):before {
  content: "DATA INÍCIO ALUG. CONTRATUAL:";
}
table[data-element-name = "table-contract-rent"] tbody tr td:nth-of-type(5):before {
  content: "DATA FIM ALUG. CONTRATUAL:";
}
table[data-element-name = "table-contract-rent"] tbody tr td:nth-of-type(6):before {
  content: "ALUGUEL CONTRATUAL:";
}
table[data-element-name = "table-contract-rent"] tbody tr td:nth-of-type(7):before {
  content: "DATA RENEGOCIAÇÃO:";
}
table[data-element-name = "table-percent-rent"] tbody tr td:nth-of-type(1):before {
  content: "SHOPPING:";
}
table[data-element-name = "table-percent-rent"] tbody tr td:nth-of-type(2):before {
  content: "CÓD. CONTRATO:";
}
table[data-element-name = "table-percent-rent"] tbody tr td:nth-of-type(3):before {
  content: "NOME FANTASIA (ATUAL):";
}
table[data-element-name = "table-percent-rent"] tbody tr td:nth-of-type(4):before {
  content: "TIPO PRODUTO:";
}
table[data-element-name = "table-percent-rent"] tbody tr td:nth-of-type(5):before {
  content: "DATA INÍCIO ALUGUEL %:";
}
table[data-element-name = "table-percent-rent"] tbody tr td:nth-of-type(6):before {
  content: "DATA FIM ALUGUEL %:";
}
table[data-element-name = "table-percent-rent"] tbody tr td:nth-of-type(7):before {
  content: "VOLUME VENDA:";
}
table[data-element-name = "table-percent-rent"] tbody tr td:nth-of-type(8):before {
  content: "% VENDA:";
}
table[data-element-name = "table-percent-rent"] tbody tr td:nth-of-type(9):before {
  content: "OBSERVAÇÃO:";
}
table[data-element-name = "table-minimum-rent-reduction"] tbody tr td:nth-of-type(1):before {
  content: "SHOPPING:";
}
table[data-element-name = "table-minimum-rent-reduction"] tbody tr td:nth-of-type(2):before {
  content: "CÓD. CONTRATO:";
}
table[data-element-name = "table-minimum-rent-reduction"] tbody tr td:nth-of-type(3):before {
  content: "NOME FANTASIA (ATUAL):";
}
table[data-element-name = "table-minimum-rent-reduction"] tbody tr td:nth-of-type(4):before {
  content: "DATA INÍCIO:";
}
table[data-element-name = "table-minimum-rent-reduction"] tbody tr td:nth-of-type(5):before {
  content: "DATA FIM:";
}
table[data-element-name = "table-minimum-rent-reduction"] tbody tr td:nth-of-type(6):before {
  content: "VALOR:";
}
table[data-element-name = "table-minimum-rent-reduction"] tbody tr td:nth-of-type(7):before {
  content: "TIPO REDUÇÃO:";
}