What is form 15111

I have been using Turbo tax for 5 years I have been a loyal customer, I'm a single mother of 3 making $27000. I received a letter from the IRS telling me I did not claim my Earned income credit, How can Turbo Tax make such a significant mistake? They advertise they are free but it never is, I was charged quite a bit for the taxes and now I get this letter, I trusted them to be diligent...

How do I fix this I cant even amend my taxes, I don't think I'll be using them again next year if I don't get some type of support. This is a big problem I see I'm not the only one by reading other posts.