newcommand{tC}[1]{@{hspace{0.1em}}#1@{}}

begin{table}[ht!]
    centering
    begin{tabular}{|tC{l}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|tC{c}|}
    {small Tonalidad} & {small 7 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$} & {small 6 $flat$}
    end{tabular}
    caption{Caption}
end{table}

Package array Error: Illegal pream-token (tC): `c' used.

documentclass{article}

usepackage{array}

begin{document}

%newcommand{tC}[1]{@{hspace{0.1em}}#1@{}}

newcolumntype{f}[1]{@{hspace{0.1em}}>{small}#1@{}}

In turn I have tried to use this new command in the creation of tables as follows:


begin{table}[ht!]
    centering
    begin{tabular}{|f{l}|*{15}{f{c}|}}
      Tonalidad &  7 $flat$ & 6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$ &  6 $flat$
    end{tabular}
    caption{Caption}
end{table}
end{document}