Why do I get the message that I must file by mail because of my "Caring for Caregiver Tax Credit on Form 511CR" credit even though I didn't receive this credit?

See if you can delete the form. 

1. Log into Turbo Tax 
2. Go to Tax tools in the left navigational pane of your return.
3. Go to tools 
4. Go to delete a form
5. Scroll down the list of forms. When you scroll to your OK forms, locate form Form 511CR.Select delete.
6. Recheck your forms to see if it has been deleted. Sign out and sign back in the return and check to see if it's been deleted.

If you using the software installed on your computer, go to the Forms mode in your return.  Locate the form and delete it. Exit the return and re-enter to see if the form is gone.