You're using a glob operator on the switch command, so you should use some form of wildcard to indicate where this string may exist in the URI. A URI will always start with a forward slash "/", so if you knew the URI started with this string, you could do this:
"/administrator*"
Otherwise your iRule should look like this:
when HTTP_REQUEST {
log local0. "uri is [HTTP::uri]"
switch -glob -- [string tolower [HTTP::uri]] {
"*administrator*" {
HTTP::respond 200 content { We are sorry, but the URL you requested is ILLEGAL. For other information, please contact IT Administrator @ XXXX }
log local0. "Request from [IP::client_addr] for [HTTP::uri] has been rejected."
}
}
}