By default, the Matomo Tag Manager is not enabled when you are running a MultiSite installation. This is because the Tag Manager doesn’t work if the Matomo plugin is network enabled. If you’re certain you won’t be activating Matomo in network mode, then you can enable it by adding below code to your wp-config.php file:

Previous FAQ: How can I configure the tracking code manually when I have WordPress network enabled in MultiSite mode?