Using [IP::addr [IP::client_addr] in HTTP_REQUEST event

is close curly bracket of /ism condition missing?

  } elseif { [HTTP::uri] starts_with "/ism" } { 
    elseif { [IP::addr [IP::client_addr] equals 10.100.128.0/18 or 10.200.128.0/18 ] } { 

There's a flaw in the logic, but not exactly sure what your intentions are. Here's the iRule reformatted approximation so you cane see what I mean:

when HTTP_REQUEST { 
    if { [HTTP::uri] starts_with "/a1" } { 
        pool a_1_HTTP 
    } elseif { [HTTP::uri] starts_with "/a2" } { 
        pool a_2 
    } elseif { [HTTP::uri] starts_with "/a3" } { 
        pool a_3 
    } elseif { [HTTP::uri] starts_with "/health" } { 
        pool a_1_HTTP_HEALTH 
    } elseif { [HTTP::uri] starts_with "/ism" } { 
        elseif { [IP::addr [IP::client_addr] equals 10.100.128.0/18 or 10.200.128.0/18 ] } { 
            pool a_1_ism
        } elseif { [HTTP::uri] starts_with "" } { 
            pool a_1_HTTP 
        } 
    } 
}

1. Right below /ism URI check you're either missing some command, or attempting to drop into a subcondition (URI starts with /ism AND the IP address matches). If the latter, you're starting a condition block with an elseif instead of an if, which is what you're seeing in the errors.

2. For the IP address check, you'd actually need to do something like this:

   { ( [IP::addr [IP::client_addr] equals 10.100.128.0/18] ) or ( [IP::addr [IP::client_addr] equals 10.200.128.0/18] ) }