Cookie Banner button not ada compliant - how to add description for accessibilty?

Hey folks!
If you look at this Accessibility Checker, you'll see that the "cookie" warning button doesn't have a description that is readable by a screen reader. 

Any idea how to force an alt tag/description onto that Cookie Banner button to fix this issue? 

Screenshot of the warning is below.  Here is the checker, so you can view the error: https://www.accessibilitychecker.org/audit/?website=oregonpatientsafety.org&flag=us 

Thanks! 
Rachel