I am doing a 2021 return with a business and cca schedule. I am trying to override the CCA to zero and the program will not let me do it. What do I need to do??

You should be overriding the Total CCA claim for the year and not any other amounts.  You'll still get a message about how it's not recommended to override, but in the case of total CCA claim, you can override the amount and still NETFILE.