The pool needs upgrading. I was able to expand the pool after the upgrade. Thank you for the help. "The vdev's assumed parity ratio does not change, so slightly less space than is expected may be reported for newly written blocks..." - I saw this in another thread. Is there a way to know what the actual available free space in the pool is? Thanks.
After updating to 7.2, I tried to expand a cache zpool. It was originally made up of 3 x 2TB nvmes. I stopped the array, added a slot to the pool (the count is now 4), selected another 2TB nvme (unformatted), then tried to start the array. I get the error "Wrong Pool State, PoolName-invalid expansion".