if the xp value is negative, it won't save.
Trainer is looking for AOB signatures of code in memory. If your version of the game has code that is being compiled differently (i.e. your computer is old or using non-AVX code, etc.) OR the code hasn't been compiled, then it may not be able to find the code.
If you are in combat, and toggle the option on, and it can't find it, then I have no suggestion.
I've got all values positive, still can't save. All stats 10, skills 275, skill xp 10200000.00, Lvl 44. How does this compare to yours?