Hi Vincent.
You posted in our Pre-Sales forum from a user account that doesn’t have any purchases associated with it.
We don’t answer support questions for Events core/free or PRO add-on in the Pre-Sales forum.
If you made your purchase(s) through a different user account, please login to that account and then post in one of the paid forums.
Thank you for your understanding and thank you for using our plugins.
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P.S. For your specific issue, it sounds like your Events Calendar ‘slug’ is set to ‘agenda’ and there’s another ‘regular WordPress page’ that has the same slug. You could just delete that WordPress page and the wp-admin message would go away (assuming this is your issue).
Reference: https://theeventscalendar.com/knowledgebase/wp-admin-settings-overview/