As long as the switches are standards based we should be able to integrate with them. We have an integration guide for use with Aruba Wireless controllers but I don't think we have a guide for integration with Aruba switches.
Are they actually Aruba switches or are they HP? I do have an old NAC and HP Procure integration guide that may be relevant?
How are you trying to activate authentication and what is the error that you see?
Keep in mind that since the device is not Extreme a lot of the automation and configuration that is built into the XMC/NAC product is not going to be able to configure the Aruba 2920 on the fly.
You'll have to enable authentication, configure RADIUS server and point it to NAC. Then build out the switch in NAC to process RADIUS, build out the rules engine, policy profiles and AVP mappings.