It seems strange.
The error is on your first post says 'description' cannot be part of FULLTEXT index.
By default there are no FULLTEXT fields in that table.
There doesn't look to be any indexes based on or containing the description field.
I've done an upgrade on a live site but it was PHP8.0.28 and DB: 5.6.51 (what ever that relates to).
Here's the create statement for that table.
CREATE TABLE `modx_site_content` (
`id` int(10) NOT NULL AUTO_INCREMENT,
`type` varchar(20) NOT NULL DEFAULT ''document'',
`contentType` varchar(50) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL DEFAULT ''text/html'',
`pagetitle` varchar(255) NOT NULL DEFAULT '''',
`longtitle` varchar(255) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL DEFAULT '''',
`description` varchar(255) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL DEFAULT '''',
`alias` varchar(245) DEFAULT '''',
`link_attributes` varchar(255) CHARACTER SET latin1 COLLATE latin1_swedish_ci NOT NULL DEFAULT '''',
`published` int(1) NOT NULL DEFAULT 0,
`pub_date` int(20) NOT NULL DEFAULT 0,
`unpub_date` int(20) NOT NULL DEFAULT 0,
`parent` int(10) NOT NULL DEFAULT 0,
`isfolder` int(1) NOT NULL DEFAULT 0,
`introtext` text DEFAULT NULL COMMENT ''Used to provide quick summary of the document'',
`content` mediumtext DEFAULT NULL,
`richtext` tinyint(1) NOT NULL DEFAULT 1,
`template` int(10) NOT NULL DEFAULT 0,
`menuindex` int(10) NOT NULL DEFAULT 0,
`searchable` int(1) NOT NULL DEFAULT 1,
`cacheable` int(1) NOT NULL DEFAULT 1,
`createdby` int(10) NOT NULL DEFAULT 0,
`createdon` int(20) NOT NULL DEFAULT 0,
`editedby` int(10) NOT NULL DEFAULT 0,
`editedon` int(20) NOT NULL DEFAULT 0,
`deleted` int(1) NOT NULL DEFAULT 0,
`deletedon` int(20) NOT NULL DEFAULT 0,
`deletedby` int(10) NOT NULL DEFAULT 0,
`publishedon` int(20) NOT NULL DEFAULT 0,
`publishedby` int(10) NOT NULL DEFAULT 0,
`menutitle` varchar(255) NOT NULL DEFAULT '''' COMMENT ''Menu title'',
`donthit` tinyint(1) NOT NULL DEFAULT 0 COMMENT ''Disable page hit count'',
`haskeywords` tinyint(1) NOT NULL DEFAULT 0 COMMENT ''has links to keywords'',
`hasmetatags` tinyint(1) NOT NULL DEFAULT 0 COMMENT ''has links to meta tags'',
`privateweb` tinyint(1) NOT NULL DEFAULT 0 COMMENT ''Private web document'',
`privatemgr` tinyint(1) NOT NULL DEFAULT 0 COMMENT ''Private manager document'',
`content_dispo` tinyint(1) NOT NULL DEFAULT 0 COMMENT ''0-inline, 1-attachment'',
`hidemenu` tinyint(1) NOT NULL DEFAULT 0 COMMENT ''Hide document from menu'',
`alias_visible` int(2) NOT NULL DEFAULT 1,
PRIMARY KEY (`id`),
KEY `id` (`id`),
KEY `parent` (`parent`),
KEY `aliasidx` (`alias`),
KEY `typeidx` (`type`)
) ENGINE=MyISAM AUTO_INCREMENT=886 DEFAULT CHARSET=utf8mb3 COLLATE=utf8mb3_general_ci COMMENT=''Contains the site document tree.'';