If I have a pretty basic Sharepoint form that looks like this. 

I want to make some customizations, nothing to complex, to this form. I want to open it in InfoPath like I have done with every other form I have created. 

This form however is a Calendar event creation form.

It's URL is provided below.

https://microsoft/Lists/AV%20Calendar/calendar.aspx

Is this the reason why I keep getting this error? 

List Settings---> Form Settings 

What are all the ways to get around this so I can open it in InfoPath and make the customizations? As you know SharePoint's form creation is pretty basic.

What makes this service "incompatible" in comparison with all the other forms. 

Thank you for time, I appreciate it!