This is a common issue that appears when you are trying to install/use/save data from an application to your PC and the username that is currently logged on doesn't have enough registry access to save the changes.
Luckily, the fix is extremely easy. You just have to run the application as Administrator. To do this, right click the main icon and choose Run as Administrator but that also implies that your username has Admin rights.
Once you do this, Li'l Red Notebook will have enough permission to write the data to the system's registry hive.