OK this is a simple fix. Your <footer></footer> tag is within the <main></main> tag and that is set to max-width: 800px; so your footer isnt ever going to be wider than 800px.
Solution: Move the <footer></footer> tag to AFTER the closing </main> tag and BEFORE the closing </body> tag like:
</main>
<!-- Footer -->
<footer>
<p>My feeble attempt to learn coding</p>
</footer>
</body>
</html>
As an aside note the <footer> should not have initially been within the <main> tag as that is for the main content of the website. The <footer> as its name would suggest and in keeping with your construction should be at the end of the html structure, so in moving the <footer> you have a much better html structure.
Of course you can have as many footers as you like for different sections of your website BUT in your case its a very simple structure so there is no dispute as to where it should be in the html code.