documentclass{article}

usepackage{booktabs}
usepackage{siunitx}

begin{document}

begin{table}[hbt!]
setlength{tabcolsep}{12pt}
centering
small
caption{The logit model label{logit}}
begin{tabular}{l
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S[table-format=2.3, table-align-text-post = false]
S[table-format=2.3, table-align-text-post = false]
S[table-format=2.3, table-align-text-post = false]
S[table-format=2.3, table-align-text-post = false]
S[table-format=2.3, table-align-text-post = false]}

           &1      &2      &3     &4       &5    &6 \ midrule
Constant   &-51    &-48    &-48   &-54     &-51  &-51 \
           & (1)   &       &      &        &     &

end{tabular}
end{table}

end{document}

! Package siunitx Error: Invalid numerical input '(1)'.