# Undefined control sequence when using citet and citep

I have issues with the commands `citet` and `citep`. The error

`undefined control sequence`

appears. As I looked up some solutions for this error, someone suggested that `natbib` is not installed.

However, if I try to load `\usepackage{natbib}`, I get the error

`Command \bibhang already defined`.

Here is the code I use:

``````\documentclass[a4paper]{article}

\usepackage[left=4cm, right=3cm, top=3cm, bottom=3cm]{geometry}

\usepackage[english]{babel}
\usepackage[utf8]{inputenc}

\usepackage{amsmath,amsfonts,amssymb}

\usepackage{graphicx}

\usepackage{booktabs}

\usepackage{biblatex}

\usepackage{csquotes}

\usepackage{natbib}

\renewcommand{\baselinestretch}{1.5}

\sloppy

``````

I use TeXmaker in conjunction with TeX Live on macOS Mojave.

By default `biblatex` only defines its own set of citation commands (including `\cite`, `\parencite`, `\textcite`, `\footcite`, ...; this is not strictly speaking exactly true: most common citation commands are actually defined by the citation style, but most styles define the same basic repertoire of commands). `biblatex` does not define the `natbib` commands `\citet` and `\citep`.
But `biblatex` has commands that do a similar thing. The `biblatex` equivalent of `\citet` is `\textcite` and the equivalent of `\citep` is `\parencite`. I usually prefer to use the `biblatex` name, a quick search-and-replace on your document should be enough. Note that `biblatex`'s `\cite` usually (certainly in the standard styles) differs from the behaviour of `\cite` with `natbib`. However `natbib` discourages `\cite` and strongly encourages use of `\citet` or `\citep` instead, so that should be less of an issue for those following this advice.
If you want to keep on using the `natbib` names even with `biblatex`, you can load `biblatex` with the option `natbib=true` to load the `natbib` compatibility mode. Note that the `natbib` compatibility mode will also change `nameyeardelim`, see Is there a disadvantage to using natbib=true with biblatex?.
In any case you can't load `biblatex` and `natbib` together, since the packages are incompatible. Adding `\usepackage{natbib}` to a preamble already loading `biblatex` will cause errors (depending on the loading order either a very clear one, or a slightly less clear one).