Braces are required around the expression, HTTP_REQUEST

Here you go, iRule Fixed. Give this a try.

when HTTP_REQUEST { 
if { [active_members Test_Pool] < 1 } { 
    HTTP::respond 200 content {   Regular Maintenance Window  
 Daily Restart of Services: 3:30 AM ??? 4:30 AM
  
During this time frame, your activities may be disrupted. Use caution working in Test System during the  maintenance window, and refrain from high risk activities such as taking or building quizzes/tests. 
**************************************************************************************************************** ***************************************** 
To all Users, 
Test System is currently down. Maintenance hours are 3:30 AM ??? 4:30 AM daily. If you encounter this page during  the referenced time frame, this is a scheduled disruption. Please try accessing Test System after 4:30 AM. 
If you are receiving this notice outside of the maintenance window, and believe you have received this message  in error, please contact the Service Center at 123-456-7890 or toll free at 800-123-4567. You may also submit your  request for support or information online by visiting the Service Center website at:  
We are working to restore service as quickly as possible and apologize for any inconvenience this downtime may  be causing you. 
Thank you. 
The Technology Services Team
} Cache-Control No-Cache Pragma No-Cache
} 
}