when entering the licence number - five groups of five digits - the form creates a minus "-" automatically after the first digit instead of a minus after the first five digits. then the licence number is detected as false.
Assuming the serial number you have for the demo/trial version begins with TRIALC- followed the groups of digits, copy the whole serial number including the TRIALC- prefix and paste this into the activation dialog.
If you are trying to enter the code in the box that has "P" beside it, that is not for the trial. Click below on the part that says "Démarrer... " and follow the instructions.