Red error messages appear in the activity log
You can view the activity log by clicking Scan on the Wordfence menu on the left of your WordPress admin console and then clicking the link "View activity log" below the "Scan Detailed Activity" box. The red errors you see here are not just Wordfence errors, but also errors from other plugins and even WordPress itself in rare cases.
If you see red errors here, they are often just warnings which may not affect the functioning of your site. But you should investigate them and report any errors to the owner of the plugin that is generating the error.
An example of a plugin error is:
Use of undefined constant user_level – assumed ‘user_level’ (8) File: /home/blah/foo/bar/home/wp-content/plugins/the-name-of-the-plugin-here/somefile.php Line: 525
If you look at the filename above, notice where the name of the plugin appears. Use that to determine which plugin is generating the errors you're seeing, and report the issue to the plugin maker. If Wordfence is generating the error, then report it to us ASAP! If the error is the last error (or close to last) that appears before a scan mysteriously stops running, then send that to us too.