Unfortunately, this is a recurring issue we simply cannot seem to put our finger on; it also happens in another course that uses this same screen.
If you want to complete this particular exercise - go through the other steps in the Candidate case - try selecting Excellent for all three categories. That is a whole number, so selecting it should get you past the not a valid decimal value complaint.
Unfortunately, my theory did not pan out; I had a suspicion it might be a localization issue. Fortunately, an esteemed colleague of mine (thank you BL) just happened to be monitoring this post and saved me from chasing after that theory. Seems this happens pretty much anywhere.
We do have another idea, but we need time to chase that down.
For now, there is no way to complete this exercise unless you reject the candidate. That is the only way to pass-by the interview step.
This does come up more often than it should so we will look into it; and I will ask (push) to make it a high-priority.
I have the same issue. In order to continue the elearning program exercices i changed the decimal values to integer without modifiying the type of properties (Example : 0.3 => 3). This helped me get around the problem. Values in next exercices may not be correct but i can continue the learning unitl the problem get resolved by Pega administrators.