Maybe this is helpful for you, even though it has been a while since you originally posted. I had the exact same problem and it drove me crazy. The first time I played the game properly a few weeks ago I got the cheats to work with no issues. Today I wanted to continue my playthrough and I couldn't activate them for the life of me. Here's what worked for me:
- type in "jassica16" during regular gameplay, all lower case, no need to press the enter key
- type at a medium speed - not too quick but also not too slow
if you type the cheat code in this way a pop-up should appear to inform you that cheats are now enabled. I tried dozens of times typing in the code at really high speed and also ultra slow, upper case, lower case, rebooted the game after every attempt etc, no dice. In the end all I had to do is type it at a regular, medium speed, sigh.
Good luck and have fun!