iRule error elseif undefined procedure

What software version are you running? I wasn't able to reproduce the issue in v14.1.2.7.

I imagine you're running into something like what's described in this other DevCentral post.

You can try restructuring your iRule as follows to see if the error goes away:

when CLIENT_ACCEPTED {
	if { [class match [IP::client_addr] equals My_NETS] } {
		pool pool_A
		snatpool snat_pool_A
	}	elseif { [IP::addr [IP::client_addr] equals 172.17.0.20] } {
		pool pool_B
		snatpool snat_pool_B
	}	elseif { [IP::addr [IP::client_addr]  equals 192.168.20.10] } {
		pool pool_C
		snatpool snat_pool_C
	}
}