wrong # of arguments"][table set monitor offline indef indef HTTP::respond 200 content "offline"]

Looks like there is also a perl script in play. As Paulius mentioned, your copy and paste isn't working.. all of the HTTP::respond should be on new lines. Also, in this example code, we are only setting table variables and responding with a status. BUT.. in testing you need to specify the correct path at the end -- somewebsite.com/offline for example.

If you are over your skis here, I always suggest engaging professional services for complex iRules.