There were no extensions found matching 'Markdown'

I can’t find some extesions,such like “Markdown”.

but,some extensions can be find like below.

Admin Tools Application

JW Player Macro2.1.10

when I click reindex, a warn log like this:
[extension.index job group daemon thread - org.xwiki.extension.index.internal.job.ExtensionIndexJob@2bc1dabe([extension, index, wiki:xwiki])] WARN o.x.e.i.i.j.ExtensionIndexJob - Failed to get remote extension from repository []: SSLHandshakeException: Received fatal alert: handshake_failure
(sory, I am allowed to post pic only one)
tomcat version is apache-tomcat-9.0.73
xwiki version is 14.10.6
help me please

It’s surprising that you have any extension at all. It seems your JVM does not support SSL certificate for some reason, but it’s strange since Let’s Encrypt is supported by Java 11 and XWiki 14.10.6 cannot run on anything older than Java 11.