And just as I finished typing my reply I have located the solution in another post.
if your cause is actionable notifications or even alexa local double check your LLAT in both. with Actionable notifications I never did add a long lived access token but I guess something somewhere along the way changed and as soon as I added the token it started working again and the unable to authenticate error went away. FYI when check the lambda_function.py script open it from the side menu not just the top or you may not see the code where you need to place the token. good luck guys
Hope this helps