Is your wallet synchronized with the block chain?
If not, then click and go to this short thread,
If the wallet is synchronized, then open the "online wallet".
Sometimes you may need to click refresh to get to the box to come up, where you can enter your passphrase.
Once the "online wallet" is open, go to the bottom just right of center and click "Start mining".
~a popup appears~
In the scroll box, scroll to and select the pool you want, then to the left side, click the "Change" button.
That automatically copies the pools Burst address to your browser.
after clicking "OK" a form will appear (sometimes takes a minute).
Paste the pools Burst address in the bottom box (Recipient), then copy and paste your wallet passphrase in the next box above that and click ok.
It takes about 16 minutes, or 4 rounds for the pool to be setup with your info, so either wait it out or don't be surprised when you see that very same error message for about 4 rounds.
Yeah in my case, and hopefully yours, the error was harmless and went away after I got things straight.
Hopefully this helps, if not 'haitch' will help you get things straightened out.
Welcome to Burst!