ContentDocumentLink example from Apex

ContentDocumentLink.apex

String ACCOUNT_ID = '001D000000K2ES6';

ContentNote n = new ContentNote();
n.title = 'My Note from Apex';
insert n;

ContentDocumentLink link = new ContentDocumentLink();
link.LinkedEntityId = ACCOUNT_ID;
link.ContentDocumentId = n.id;
link.ShareType = 'V';
link.Visibility = 'AllUsers';
insert link;

I tried this with adding a note to an event but get the error
"First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [Document]: [Document]".

My code is:

    ContentNote cn = new ContentNote();
    cn.content=Blob.valueOf(noteforevent.escapeHTML4());
    cn.title='Note on event';
    
    ContentDocumentLink link = new ContentDocumentLink();
    link.LinkedEntityId = eventID;  //eventId is coming in with valid content!!
    link.ContentDocumentId = cn.id;
    link.ShareType = 'C';
    link.Visibility = 'AllUsers';
    insert link;

Enabling this triggers gives error to community user while creating Notes.