Questions about TEX when I try to Constuct a tablet

Excuse ME, when I try to construct a tablet in LaTeX, I got the following error message:

!Missing \endgroup inserted.
<inserted text>
\endgroup
1.141  & \multicolumn{2}{c|c|}{Cash Flow at}
\\ \cline{3-4}


Here is my original code in LaTeX. Could you please tell me How can I revise it to avoid the error?

\begin{tabular}{|c|c|r|r|}
\hline
& \multicolumn{2}{c|c|}{Cash Flow at} \\ \cline{3-4}
\textbf{Action} & \textbf{Cash flow at $t=0$} & \textbf{$t=3$} & \textbf{$t=6$} \\ \hline
Lend \$3,000,000 in 3-years markets & -\$3,000,000 & +\$3,649,428.891 &0 \\ \hline Roll over at$t=3$for 3 years & 0 & -\$3,649,428.891 & +\$3,649,428.891+\tilde y\\ \hline Borrow PV(\$3,649,428.891) in 6-years markets  & +\$3,100,554.785 &0 &-\$3,649,428.891 \\ \hline
\textbf{Total} & +\$100,554.7855 &0 & +\tilde y \\ \hline \end{tabular} \\  1 Answer In you example there are basically two problems: 1. The error is due to \multicolumn{2}{c|c|}{Cash Flow at} in which only one column specifier is required as second argument, which should be \multicolumn{2}{c|}{Cash Flow at}. 2. Another problem is \tilde y which should be inside $...$ symbols, i.e. $\tilde{y}$. The corrected MWE will be: \documentclass{article} \usepackage{geometry} \begin{document} \begin{tabular}{|c|c|r|r|} \hline && \multicolumn{2}{c|}{Cash Flow at} \\ \cline{3-4} \textbf{Action} & \textbf{Cash flow at$t=0$} & \textbf{$t=3$} & \textbf{$t=6$} \\ \hline Lend \$3,000,000 in 3-years markets & -\$3,000,000 & +\$3,649,428.891  &0 \\  \hline
Roll over at $t=3$ for 3 years   & 0 &  -\$3,649,428.891 & +\$3,649,428.891+$\tilde {y}$\\ \hline
Borrow PV(\$3,649,428.891) in 6-years markets & +\$3,100,554.785 &0 &-\$3,649,428.891 \\ \hline \textbf{Total} & +\$100,554.7855 &0  & +$\tilde {y}$  \\  \hline
\end{tabular}

\end{document}