I have the update XML shown below.
The last update I published was with Joomla! 3.6, so the target platform was 3.6 I now changed it to 3.[6789] to support all versions since 3.6. But the update does not show up.
For clarification, I have the version 3.0.0 installed and try to update to 3.1.0.
I used https://docs.joomla.org/Deploying_an_Update_Server for creating the xml.
<updates>
<update>
<name>SchuWeb Sitemap</name>
<description>SchuWeb Sitemap - Sitemap Generator for Joomla!</description>
<element>com_schuweb_sitemap</element>
<type>package</type>
<version>3.1.0</version>
<infourl title="SchuWeb Sitemap 3.1.0">https://extensions.schultschik.com/products/schuweb-sitemap/schuweb-sitemap-3-1-0</infourl>
<downloads>
<downloadurl type="full" format="zip">https://github.com/svanschu/SchuWeb-Sitemap/releases/download/3.1.0/pkg_schuweb_sitemap_3.1.0_20200417.zip</downloadurl>
</downloads>
<tags>
<tag>stable</tag>
</tags>
<maintainer>Sven Schultschik</maintainer>
<maintainerurl>http://www.schultschik.de</maintainerurl>
<section>frs</section>
<targetplatform name="joomla" version="3.[6789]" />
<php_minimum>5.6</php_minimum>
</update>
...
Link to the complete update xml: https://raw.githubusercontent.com/svanschu/SchuWeb-Sitemap/master/update/schuweb_sitemap.xml
What am I missing, I don't see the mistake ...