I just resolved the exact same problem.
Download and install this action scheduler plugin:
https://github.com/woocommerce/action-scheduler/archive/3.1.4.zip
Then use that plugin to create the data base tables related to the action scheduler plugin simply by loading the interface via wp-admin:
https://yoursite.com/wp-admin/tools.php?page=action-scheduler
You’ll see a notification banner at the top indicated the service will try to recreate the missing tables for the action scheduler.
Once that’s done, try the WP Mail SMTP plugin again.
