Received a CP301 letter from IRS, saying that I owe back taxes. Why didnt Turbotax catch this when I'd filed taxes with them?

[VolvoGirl]

When you did your 2019 return?  Turbo Tax doesn't know anything about you owing back taxes.  The IRS doesn't tell Turbo Tax anything.  Each year is separate.  Or did you file the past years with Turbo Tax?  Then it was up to you to pay the tax due.  Turbo Tax doesn't know if or when you paid it. 

If you filed the returns and the tax due was suppose to come out of your bank account but maybe you entered the account number wrong then you should have noticed that the payment never came out. 

Or did the IRS change your tax returns so you owe more?

[ee-ea]

A CP301 notice is not about owing money to the IRS. See the explanation here

https://www.irs.gov/individuals/understanding-your-cp301-notice