How to fix “Underfull \hbox (badness 10000)” warning?

Instead of forcing an underfull box with \newline, you could simply leave an empty line to start a new paragraph:

\documentclass{article}
\usepackage[T2A]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{titling}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{tikz}


\setlength{\droptitle}{-3.5cm}
\setlength{\parindent}{0cm}
\newcommand{\squad}{\hspace{0.5em}}

\author{vladgovor77771}
\title{Some article}

\begin{document}
\maketitle

\textbf{Task 1} 

Task description: 

blabla

\end{document}