These symbols are not defined by default in LaTeX: you have to load some additional packages. I think adding

\usepackage{marvosym}
\usepackage{amssymb}

$\blacksquare$

i got this messege:

LaTeX Error: File `marvosym.sty' not found.
Emergency stop.

Try to install the texlive-fonts-recommended package (see Ubuntu search page).

thank you.
i sloved the problem by added:

\usepackage{amssymb}
into the head

\newcommand{\tickYes}{\checkmark}
$\tickYes$
\newcommand{\tickno}{\times}
$\tickno$
\newcommand{\tickblk}{\blacksquare}
$\tickblk$

inside the document.

regards

sorry this is the best:

\usepackage{amssymb} % into the head

the past this in any line:

\mbox{\mathsurround=0pt \makebox[0pt][l]{$\checkmark$}$\square$}
\mbox{\mathsurround=0pt \makebox[0pt][l]{$\times$}$\square$}
\mbox{\mathsurround=0pt \makebox[0pt][l]{$\blacksquare$}$\square$}