Thank you Corion for your guidance. I solved this issue with below metthod:-

1. Without declraing width of multicolumn.

\\ {\bf [% tx.serial %]} & {\bf [% tx.type %]} & [% tx.item_product_code %] &  [% tx.dc %] & [% tx.currency  %] & [% tx.amount %]\\

\multicolumn{6}{c}\bf [% tx.comments %] \\

2. with declaring width of multicolumn for content wraapper, by declaring table inside multicolumn

\\ {\bf [% tx.serial %]} & {\bf [% tx.type %]} & [% tx.item_product_code %] &  [% tx.dc %] & [% tx.currency  %] & [% tx.amount %]\\

\multicolumn{5}{l}{\begin{tabular}[t]{p{4in}}

   Details: [% tx.comments %]

     \end{tabular}}\\