Good evening James and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the error message you are receiving during install. Yikes!
I would love to help you with this topic.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
Secondly, could you tell me from which version you were upgrading from ?
Finally, can you tell me if the message disappears when you re-save the recurring events individually ?
Hang in there,
Best regards,
Geoff B.