A reason for not being able to assign a monitor to a pool may be, that your pool members are configured to use a wildcard port of "0".
A default "tcp" or "tcp_half_open" monitor relies on the preconfigured ports of your poolmembers. With a wildcard port it simply doesn´t know which port to address and is not available for selection.
Other monitors i.e. "icmp" or "tcp_echo" are available to be used with wildcard port configuration.
Alternatively you may want to configure a custom monitor with a specific service port which can be used with poolmembers on port "0" as well.
Did you notice any messages in /var/log/ltm when trying to assign the monitor?