The trial activation key is 30 characters long, but the five boxes to place activation key only holds 25 characters. What's with that??
You might wanna open up a ticket with them so that your concern will be addressed Its in the Support link above. Hope this helps.
Hello, Please make sure the version of the trial key is the same as Parallels Desktop software you are trying to activate. Also please make sure you are not using Parallels Desktop trial key to activate operating system of your virtual machine. Please provide a screenshot if possible.