@ChadinTX
The best way to review what the root cause on this is to look at the Summary page via Access Tracker, to see what type of auth method is being applied.
Since the Days-Since-Auth is typically used with a Captive Portal (L3) and MAC Auth (L2) authentication. Any L2 auth will attempt to happen before L3 Auth. If the user has not passed the captive portal authentication, or the retention rate for insightDB was not extended and the data was purged (depends how many days you are doing mac caching for). We will see this error.
In the end it just means that the data was not presented via the insightDB, and the only way to get this data inside insightDB is to have the user pass captive portal authentication.
I have included a screenshot that I had generated in the past via insightDB, and you can see that Item #3 shows the email address (backed out) for both username fields. This is where I was able to validate that the L3 auth was being passed. Once the L3 auth was in place, and insightDB has the data the user should not get any errors about the days-since-auth not being calculated.
I dont have the exact colums as what I used to make up the report, although with a little work you can probably take this data and go through the options and review what would be needed with the client info from Access Tracker and this to build your report.
Hope this info helps!
Justin