Troubleshooting 

Set a class in the constant defaultCssClass first. It is added to every page, so you can see whether the extension renders the body tag at all. Reload the frontend and check the source code after every step.

  1. Make sure the extension is installed and the database fields exist (Admin Tools > Maintenance > Analyze Database Structure).
  2. Make sure the static template Pagewide CSS Classes (pageclasses) is included in your main TypoScript template (see Include the TypoScript). Check it in the Web > Template module: Template Analyzer in TYPO3 11, Included TypoScript in TYPO3 12.
  3. page.bodyTagCObject may be set by another extension or by your own TypoScript. Find out with the TypoScript Object Browser (TYPO3 11) or Active TypoScript (TYPO3 12) and assign the plugin at the very end of your setup, see Manual setup.
  4. Your PAGE object might not be called page. Check this in the same module.
Template Analyzer listing the static template "Pagewide CSS Classes (pageclasses)"

Template Analyzer (TYPO3 9-11): the static template of the extension is included.

TypoScript Object Browser showing page.bodyTagCObject set to the pageclasses plugin

TypoScript Object Browser (TYPO3 9-11): page.bodyTagCObject is the plugin of the extension.