If you enter any numbers into the cpp boxes and saved and if you later change make the box blank and check off "Exempt CPP/QPP" it will NOT work.
Create a new T4 form with a temporary employer name the ("+" bubble), copy all the T4 box amounts to the new form (Leave CPP boxes blank ie. don't touch). Then select "Exempt CPP/QPP (rare!)".
Save the new form, delete the old first form (click on "-" bubble).
Rename the new T4 name properly
The software will now calculate properly and remove the error such you can Netfile.