not allowed in tag?

I'm in the process of cleaning up a website's code. One of the things I am doing is running each page's code through the W3C Markup Validation Service: https://validator.w3.org/

For the page I'm currently working on, the validator gave me a lot of errors. I have already fixed nearly all of them but there is one error I can't figure out: Line 92, Column 108: document type does not allow element "select" here; missing one of "p", "h1", "h2", "h3", "h4", "h5", "h6", "div", "pre", "address", "fieldset", "ins", "del" start-tag

The section of code that is giving me this error is here (line 92 is the one starting with select size="1" and column 108 appears to be just before the fullstop right after Pageselector):

<div style="text-align:center;">
    <form id="Pageselector" action="">
        <select size="1" name="FC" onchange="location.href=Pageselector.FC.options[selectedIndex].value">
            <option selected="selected">Go to Page...</option>
            <option value="AC_1.html">Page 1: Introduction</option>
            <option value="AC_2.html">Page 2: Essential Optimization</option>
            <option value="AC_3.html">Page 3: Troubleshooting Tips</option>
            <option value="AC_4.html">Page 4: Patches &amp; Mods</option>
            <option value="AC_5.html">Page 5: In-Game Settings</option>
            <option value="AC_6.html">Page 6: In-Game Settings (Pt.2)</option>
            <option value="AC_7.html">Page 7: In-Game Settings (Pt.3)</option>
            <option value="AC_8.html">Page 8: Advanced Tweaking</option>
            <option value="AC_9.html">Page 9: Adv. Tweaking (Pt.2)</option>
            <option value="AC_10.html">Page 10: Neat Stuff &amp; Concl.</option>
        </select>
    </form>
</div>

The code in question is a drop down list that lets you select which web page in the series you want to look at. After choosing an option, the browser opens the selected page. I don't understand what the problem is because according to w3schools, the select element is allowed to be inside a form element: https://www.w3schools.com/tags/tag_form.asp

If you need to see all the code on the page or need anything else, you can say so and I will edit this question. The DocType I am using is XHTML 1.0 Strict. The code above does do what it is supposed to. I'm just trying to clean and refactor it.