\documentclass{article}
\begin{document}
Bla bla bla. Figure 1 presents the results of the overall period: \\
{\centering [Insert Figure 1 here] \par} \\
Overall, the model seems to work well bla bla bla
\end{document}
The first \\ produces the warning
Underfull \hbox (badness 10000) in paragraph at lines 5--6
which is the maximum level of badness reported by TeX. Never place a \\ at the end of a paragraph, it does not add vertical space it just forces the paragraph to have a spurious extra line with no content (hence the warning).
The second \\ generates an error message,
! LaTeX Error: There's no line here to end.
Never ignore errors, the way TeX recovers from errors is only designed to allow the rest of the file to be checked for further errors, it is not designed to make sensible pdf output. The error here is because \\ is following \par so specifying a line to end before the paragraph has started,

\documentclass{article}
\begin{document}
Bla bla bla. Figure 1 presents the results of the overall period:
\begin{center}
[Insert Figure 1 here]
\end{center}
Overall, the model seems to work well bla bla bla
\end{document}
\\to end a paragraph! Usecenterenvironment instead of\centering. It already adds vertical space. Or (better) use afigureenvironment and let the figure float. – Schweinebacke May 24 '17 at 16:23\\will have tex complaining about underfull hbox badness 10000, which is the maximum badness. remove the\\the\centeringand the\parand use\begin{center}which adds suitable space. – David Carlisle May 24 '17 at 16:31