TurboTax will automatically calculate your CPP over payment, if you have one. You can see what it calculates on the bottom of the Detailed Summary. This FAQ shows how to get there: https://turbotax.community.intuit.ca/community/tax-return-status/help/how-can-i-see-a-detailed-summa...

T1-General Line 44800 is 0.00 despite an overpayment of 48.06 showing on Schedule 8 - Line 12 

There is no option to override this, even though it is clearly an error.  

Please help correct this, it will not allow me to override box 44800 to input the overpayment.

You have not provided enough information so we can help you. If you are filing prior years' returns, there is a limit from CRA on how far back you can go to claim the CPP and EI overpayment. CPP can go back 4 years, and EI can go back 3 years. 

I have attached a copy of my duplicate of your problem in a 2019 version. I was able to claim a CPP overpayment. 

I hope this was helpful