Downloading football schedule to calendar

When trying to use my iPhone XR calendar to subscribe to an organization's public calendar, I get an "Insecure Connection" dialogue box, then this error message:

Dialogue box: "Insecure Connection The connection is not secure. Do you want to continue subscribing"

Error message: "Validation failed. please edit the url and try again."

What does this mean?

Is it something I can correct? or . . .

Is it a problem the organization needs to correct on their website?