Hi franco.demei, welcome to Thunkable! 
Be sure to check out How to ask Great Questions v2.0, the Community Guidelines, and our Getting Started Guide to make the best of your Thunkable Community experience!
It is always best to use the green error block and attach it to a label to see what the specific error is in these cases. It will be impossible for us to help otherwise. I am also assuming there is no UUID in the blocks for security reasons (you don’t want to show us) but if not, that characteristic UUID is required anytime you connect with or transmit data to and from a BLE device.