With the very appreciated help from Harald, the Modules problems I had in the previous posts are fixed.
I did a few other things on the site and also started to create the Sandbox, using the manual provided with the Braintree Module. I checked several times that the settings in the payment module are the same as the one I set up in the sandbox.
Excited as I was .... logged into the store after creating a 10 cent product and tried to make a sandbox payment.... and got this error message, no matter whatever card I used
The card number was not able to be processed. Please try again and if problems persist, please try another payment method.
Does anyone have an idea what is going wrong ?