How can I integrate my slack to redash.

I am unable to integrate slack bot for posting visualization on slack.
I follow the extract steps as define on your help page.
I also rechecked everything, It still unable to connect with redash.
It showing :
Failed validating credentials please make sure you used the correct API key & your Redash instance is accessible.
If you want to start again just tell me “start setup”.
And my redash instance is publicky available also.

Please help me out.