PHP - Exec function, shell_exec, and system do not work correctly

Running the example documentation on the site itself works.

Examples:

echo exec('whoami');

$output = shell_exec('ls -l');
echo "<pre>$output</pre>";

My code does not differ so much from the examples described in the documentation, what I want to do is just run another application with the following command:

My code / example:

> $comando = "python3 caminho/arquivo.py";
> exec($comando);

And now folks!?