The nodes in the enclosure being added are used elsewhere in the cluster, the target I/O group contains nodes of a different control enclosure, or another enclosure has not yet completed the process of being added.
If a node already exists in the I/O group, add nodes from the same enclosure only. If the I/O group is empty, you can use a different control enclosure in which the nodes are not in a cluster. If you are currently adding another enclosure, wait for the completion of that process. Ensure that both nodes of an added enclosure are online and that the enclosure is listed in the output of the lsenclosure command.