Hi Alexandra,
Thanks so much for reaching out!
Our plugin is designed to work with simple products, so I cannot guarantee compatibility with subscription products.
If everything works, with the exception of the number of tickets available, you can hide the number of tickets available so as not to confuse your users.
Also, does this only occur when you have a bare WordPress install (a default WordPress theme, and only our plugins and Subscriptions installed)?
Can you please enable WP_DEBUG and WP_DEBUG_LOG and share any debug messages you see?
Let me know if you have any other questions in the meantime!
Thanks,
Jaime