Good evening Catherine and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the error 500 message you are getting after clicking on “Add to cart”.
I would love to help you with this topic.
I have tried reproducing this issue on Chrome while using both a Mac and a PC and I was not able to unfortunately.
Is it possible you have changed anything since you posted this question ?
Secondly, could you tell me which version of Chrome and Windows you are using ?
Best regards,
Geoff B.