Assuming you are referring to the BRAND name 'Sense" energy monitor. It needs to connect to the voltage and have the current pass through the current transformers (CTs). So you need a 2 pole circuit breaker (to protect itself) at the same location as the main incoming power for the CTs. So putting it at one of the sub-panels would be problematic, unless you only want to monitor the power consumed off of that sub-panel.
But without knowing everything, why do you have a sub-panel for the pool, then you have another breaker that says "Pool E.Q."? If whatever the E.Q. is can be powered FROM that same sub-panel, that would free up a spot in your main panel.
Alternative 2 is that you would get a "quad" breaker set, 4 poles that connect in a 2 pole space.