I do have two Radio RA2 systems, one at work (the one your trying to register) and one at home. They are both linked to the same email address and both appear within the app. The app supports multiple systems. Can you contact Lutron to see if this could be part of the problem?
To verify suspicions, was the Repeater you are looking to link added as the “Add a Home” device (not the original repeater the account was registered for)?
I have looked into this issue and it appears that due to the method of activating secondary RadioRa 2 systems within the Home Control+ app, it is not currently possible to individually link repeaters that were added into the Home Control+ app as additional home devices. I submitted a request to develop a solution for this circumstance.
To link the desired serial number to your Alarm.com account, it will need to use its own Home Control+ login name. It appears that the primary Lutron repeater in a multi-system setup through the Home Control+ app can also be successfully linked.