How do I close notes on the right side of my Calendar?

Wondering if Apple support's online article for MacOS Mojave can help:

See: Add notes, a URL, or files to events in Calendar on Mac - Apple Support

Eg" says:

: .... You can attach files, such as a form for an appointment or a document to discuss at a meeting".

and:

• "Attach a file to an event: Click Attachments, then select a file to attach. You can also drag a file to the event info window.
• Preview an attached file: Select the file, then press the Space bar.
• Remove an attached file: Select the file, then press the Delete key."

Resource: Calendar User Guide for Mac - Apple Support