Hi Ashraf, I hope your problem has been resolved. I was able to recreate the issue you’re having (see image below).
Leaving the following attribute empty (see image below) caused the error.
I suspect that the input parameter in your microflow “DocumentToUpload” hasn’t set a value for the “Key” attribute prior to passing it to the microflow from your screenshot.
I think you are getting the empty Key value which means, there is no value available in the credential object. Check out that action.