The following modification solved the error about SeverityMessage not defined when displaying logs in debug mode.
This doesn't resolve the root cause of the issue which is the creation of the log when displaying the Dashboard.
1- Open file src/PrestaShopBundle/Resources/views/Admin/Common/Grid/Columns/Content/severity_level.html.twig
2- Search line #42 {% set severityClass = '' %}
3- Add after {% set severityMessage = '' %}
Source :
https://build.prestashop.com/news/coreweekly-14-2021/ check #23856 that redirects to github