XML::htmlParse parses html in the form of a character string. You are trying to get XML::htmlParse to parse an object of class "response" from the httr package. XML doesn't know what to do with this object.
However, the error you are getting seems to be a curl error rather than an XML error. Depending on your platform, it may be easiest to just use a different method to obtain the html.
Instead you can try:
url <- "https://www.cagematch.net/?id=1&nr=283492"
XML::htmlParse(paste(readLines(url), collapse = "\n"))
#> <!DOCTYPE HTML PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
#> <html xmlns="http://www.w3.org/1999/xhtml">
#> <head>
#> <link href="/2k16/css/2k16.css?20200712" rel="stylesheet" type="text/css">
#> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
#> <meta http-equiv="Content-Language" content="en">
#> <meta http-equiv="Content-Security-Policy-Report-Only" content="require-trusted-types-for 'script'; default-src 'self' fonts.gstatic.com; script-src 'self' fonts.gstatic.com; connect-src 'self'; img-src 'self' www.paypalobjects.com; style-src 'unsafe-inline';base-uri 'self';form-action 'self';object-src 'none'">
#> <meta name="viewport" content="width=1120">
#> <meta name="description" content="Internet Wrestling Database">
#> <meta name="keywords" content="wrestling,wwe,raw,smackdown,wrestlemania,aew,dynamite,impact,tna,wcw,ecw,roh,wwf,njpw,ajpw,puroresu,wrestling database,wrestling news,wrestler,superstar">
#> <meta name="author" content="Philip Kreikenbohm">
#> <title>ATP « Events Database « CAGEMATCH - The Internet Wrestling Database</title>
#> <script language="JavaScript" id="erasable" type="text/javascript" defer>
#> window.onload = function() { loadComments("commentBox", "1", "283492", "", "en"); }
#> </script>
#> </head>
#> <body class="TemplateBody">
#> <div class="LayoutUserAccount LayoutWidth">
#> <a href="https://www.cagematch.net/de/"><img src="/2k16/img/german.png" class="LayoutLanguage" alt="Deutsch" title="Deutsch"></a><a href="https://www.cagematch.net/en/"><img src="/2k16/img/english.png" class="LayoutLanguage" alt="English" title="English"></a><div class="UserHeader">Not logged in or registered. | <a href="?id=872">Log In</a> | <a href="?id=871">Register</a> | <a href="?id=879">Password lost?</a>
#> </div>
#> </div>
#> <div class="LayoutHeader">
#> <div class="LayoutMainHeader LayoutWidth">
#> <div class="LayoutLogo">
#> <a href="?id="><img class="HeaderLogoLeft" src="/2k16/img/header/header2.webp" alt="CAGEMATCH Logo" style="width:570px;height:100px;" title="CAGEMATCH"></a>
#> </div>
#> <div class="LayoutSearch"><form action="" method="get" id="Search">
#> <input name="id" type="hidden" value="666"><input type="text" name="search" class="HeaderSearchInput" value="Search the site..." onclick="changeStateHeaderSearchBar(this,'en')" onblur="changeStateHeaderSearchBar(this,'en')"><input type="submit" class="HeaderSearchButton" value="Search">
#> </form></div>
...etc
