LaTeX ERROR: "Missing number, treated as zero" & "Illegal unit of measure (pt inserted)."

Question

I can't solve these problems with my latex project. In particular, the errors are:

Missing number, treated as zero. Illegal unit of measure (pt inserted).

I tried to comment every 'suspect package' in my document but anything changed. This is my preamble:

\documentclass[11pt, twocolumn, a4paper]{article}
\usepackage[colorlinks, linkcolor=black]{hyperref}
\usepackage[italian]{babel}
\usepackage[utf8]{inputenc}
\usepackage[landscape, margin=1.5 cm]{geometry}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{calc}
\usepackage{fancyhdr}
\usepackage{capt-of}
\usepackage{booktabs}
\usepackage{fontenc}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\pagenumbering{arabic}
\setlength{\columnsep}{1cm}
\pagestyle{fancy}
\fancyhead{}
\fancyhead[L]{\rightmark}
\fancyfoot{}
\fancyfoot[R]{\thepage\ }
\renewcommand{\headrulewidth}{}
\renewcommand{\footrulewidth}{}
\renewcommand{\thefigure}{\arabic{figure}}
\renewcommand{\thetable}{\arabic{table}}
\renewcommand{\familydefault}{\sfdefault}
\fontfamily{qag}\selectfont

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}
\begin{titlepage}
%my personal template title
\end{titlepage}
\end{document}

Problems appear with commands: (i) \newpage , (ii) \subsubsection and (iii) \paragraph.

Thank you for any possible answers.

Answer 1

You can't redefine

\renewcommand{\headrulewidth}{}
\renewcommand{\footrulewidth}{}

to be empty. If you want to remove the head- and footrule, set their widths to zero instead:

\documentclass[11pt, twocolumn, a4paper]{article}
\usepackage[colorlinks, linkcolor=black]{hyperref}
\usepackage[italian]{babel}
\usepackage[utf8]{inputenc}
\usepackage[landscape, margin=1.5 cm]{geometry}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage{calc}
\usepackage{fancyhdr}
\usepackage{capt-of}
\usepackage{booktabs}
\usepackage{fontenc}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
\pagenumbering{arabic}
\setlength{\columnsep}{1cm}
\pagestyle{fancy}
\fancyhead{}
\fancyhead[L]{\rightmark}
\fancyfoot{}
\fancyfoot[R]{\thepage\ }
\renewcommand{\headrulewidth}{0pt}
\renewcommand{\footrulewidth}{0pt}
\renewcommand{\thefigure}{\arabic{figure}}
\renewcommand{\thetable}{\arabic{table}}
\renewcommand{\familydefault}{\sfdefault}
\fontfamily{qag}\selectfont

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\begin{document}
\begin{titlepage}
my personal template title
\end{titlepage}
\newpage
test
\end{document}