Hmm, I got some sort of solution.
At the permalinks-settings, I unchecked the “Plain” setting and instead, activated the “Post-name” setting.
Now, Yoast created a sitemap https://studieboekenhuis.nl/sitemap_index.xml
I tried to add that address on Google Search Control, but now, Google can’t find that sitemap. Google does find my sitemap when I type https://studieboekenhuis.nl/index.php?xml_sitemap=params=
So my question is : does Google visit my sitemap or not ?
