An array manager can be deleted when no devices are in SRM protection groups from any array pair in that manager and no array pairs are enabled.
First ensure no devices are protected (there should be no protection groups listed in the Discover Devices columns for any enabled array pairs in the array manager pair):
Then disable any array pairs:
Then select any disabled pair and then Array Manager Pair and choose Remove:
SRM will confirm the array pairs related to that array manager pair before proceeding:
Click Remove. If it fails, it means you missed an array pair: