What exercise and what step are you doing in the exercise? Exactly what have you typed and in which fields of the form? Does the error happen when you hit ok in the wizard? Perhaps you could upload a screenshot.
I have this exact error at the same step. I spent hours trying to figure out what is going wrong and have no luck.
In the case, I select the assignment step "Select New Position", when I go to Configure form the error message is displaying highlighted red:
The name you entered is not valid.
I follow the steps in the solution but the text will not go away. Obviously it will not save either.
Text from the solution below:
1. Open the process outline for the Qualify Step by clicking Configure process detail from the Case Designer stages view. Select the Determine Target Position multi-step process and then click the Select New Position assignment.
2. Click Configure form for the Select New Position action.
3. Add a PositionAppliedFor text mode property and a Position page property. All fields in the Position data page should be Read Only
4. Click Save to close the Form Configuration dialog.