Actually we face this error sometime when we try to add Google Adsence gadget.
I have some simple steps for solve this error, just follow these simple steps and look it what happen.
your problem was solve.
Please Follow these Steps:
Log-in to your Google Adsense account
Create an ad unit that you want to add into your blog (i.e. Link Unit)
Get the code for the Google AdSense ad unit
Copy the code.
Then sign-in to your blogger
Go to Layout
Click on Add New Gadget
And wait for 20 30 minutes after than you Google Adsence ad previews in your blog.