Safari can't open the page because the address is invalid.

Use a valid page address instead.

Did you follow a link, or copy-pasted a URL, or did you type it in manually?

Did you get a status code from the remote server? Or did the browser put up an error pop-up message?

Copy-pasting can accidentally omit a character, e.g. ttps://www.apple.com instead of https://www.apple.com, making the address invalid.

A link from someone els might be invalid, if the source creator made a mistake.

Else it is quite difficult to comment without seeing and trying the address.