Adding www to url results in Not Found error

It’s a server thing. You need to redirect https://www.nyac.org.au/ to https://nyac.org.au/ – probably via your root .htaccess file.

Thanks esmi,
Sorry for my ignorance, but do you mean the root directory of the site or the root directory of the server?

And what code would I need to do something like this?

The root directory of the site. I’m assuming it’s in the root domain folder, yes?

See https://stackoverflow.com/questions/234723/generic-htaccess-redirect-www-to-non-www

Do ensure that these lines are added above the WordPress block in the file (if that block exists).

I have the site in a folder in the root directory.

I tried putting this

RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} !^www\.(.*)$ [NC]
RewriteRule ^(.*)$ https://www.%{HTTP_HOST}/$1 [R=301,L]

in the .htaccess files at both the root level and in the folder. Both together and separately (trying everything here!) But just got the same not found error or the Plesk page in all cases.

Is there something the sysadmin can do?

I think you need to have a word with them, yes.