I encountered the same problem. What I found was that somehow the LLC had been marked as disregarded on last year's return. What I had to do is go into TT 2017 and amend the return to NOT check U(1) "disregarded" etc. Then (unfortunately) I had to start the entire process of preparing my 2018 return all over again, this time identifying with the newly modified 2017 TT tax file. Once I got to the California review, I was asked to clarify whether this was a single member LLC once again ("the box must be checked") but found that now I was being prompted because just below it the single owner "type" box under "Single Member LLC Information and Consent" (i.e. "individual", "C Corporation" etc.) had to be unchecked. After I unchecked the single owner type box, and ensured U(1) was unchecked, and ensured the worksheet did not include any indication this was a single member LLC, it passed the error check and I was able to e-file. Hope that helps.
