Explanation
If you find yourself unable to navigate to an object or visual while using a keyboard, it may be because the report author has decided to hide that object from the tab order. Report authors commonly hide decorative objects from the tab order. If you find that you cannot tab through a report in a logical manner, you should contact the report author. Report authors can set the tab order for objects and visuals.
Reference:
https://docs.microsoft.com/en-us/power-bi/create-reports/desktop-accessibility-consuming-tools