to qualify for the septic credit, do I need to file schedule CMS?

Is this for Massachusetts?  If so, yes, but TurboTax will complete Schedule CMS when you enter your expenses.

To claim the Septic Credit, you: 1) Must be the owner of the residential property located in Massachusetts; 2) Must occupy the property as your principal residence; and 3) May not be the dependent of another taxpayer.  

You must complete Massachusetts Schedule SC and retain it for your records along with a Certificate of Compliance.

If you voluntarily repair or replace a cesspool or septic tank, this credit is not available since it is not considered a "failed" system.

Note: Each Certificate of Compliance or verification letter requires its own Schedule SC to claim a credit. TurboTax only supports one Septic Credit, current or prior year, per return.