Using WP Matomo on Wordpress 5.6 shows the following error in the browser console:
Uncaught TypeError: jQuery(...).size is not a function

Clicking through the debugger i found out that it's caused by the Tour plugin:
https://github.com/matomo-org/wp-matomo/blob/c7b6b0defd115199ad5a71d748d46db9b5a38b51/app/plugins/Tour/templates/engagement.twig#L53

The documentation shows that the .size() method has been removed in jQuery 3, so it should be replaced by the .length attribute:
https://api.jquery.com/size/

This change resolved the error, but i did not test the Tour plugin as a i don't know how it is supposed to work.

Best regards,
Malte