This is a standard LaTeX error which appears when you do not include a float specifier in the options of a float i.e. the ht in \begin{figure}[ht]. The float specifier parameter is an option which allows us to have a greater control over where a figure is placed. The different placement options available to us are listed below:

These float specifiers can be placed inside the square brackets when we create a float, such as \begin{figure}[h]. They can even be placed in combinations such as \begin{figure}[!htb], which tells LaTeX to place the figure here, or top, or bottom. If you leave the square brackets blank by writing something like \begin{table}[], an error message will appear such as the one shown below.

How to resolve the error

Removing the square brackets:

The simplest way to get rid of this error is to remove the square brackets when we are not using a float specifier i.e. instead of writing \begin{figure}[], you could simply write \begin{figure}.

Inserting a float specifier:

Another way to resolve this error is by inserting a float specifier telling LaTeX where you would like your float to appear. This should be included in the square brackets, such as \begin{figure}[h] or \begin{table}[!htb].