To recover from this situation use the following procedure:
When attempting to deploy the LAG the following error message was received:
Session 12 in secondary port (15/4) of this LAG: Deploy Failed
Reference the sessionnumber (12)in the error message.
Open the same number of sessions as indicated in the error message, 12 in this case; then enter configuration mode in the 12th session and go into the port 15/4 configuration level; then exit cleanly from configuration mode using the exit or end commands.
This will remove the session lock from the port; after this, you should be able to add the port to the LAG and deploy it to complete the configuration.