Please check your server configuration. You may want to whitelist “*.html” files from the “plugins” directory. The HTTP status code is 403 for URL “plugins/SitesManager/templates/index.html”
It seems like your server is returning a 403 Forbidden when Matomo tries to load the plugins/SitesManager/templates/index.html file.
It is very likely that you installed Matomo in a subdirectory of the main site and the .htaccess file in the main directory includes rules that block access to .html files.