You are correct, you may use either your SSN or the EIN of your SMLLC on your K-1. Please go back to the place where you entered the information about your LLC and check to see whether you may have used your EIN anywhere. If you are using a Schedule C or any other forms in connection with your business, please double check. If you did use the EIN, TurboTax is trying to map that EIN with your K-1, and creating an error.
Here is more information supporting the use of an SSN by a SMLLC.
@WagnerFamilyPTR
