You cannot transfer informtation from an 1120 to an 1120s within the same tax year. This conversion can only happen automatically in the process of transferring the return from a prior year into the current year.
In other words, a 2017 1120 may be transferred into a 2018 1120s file, but you cannot take a 2018 1120 and convert it into a 2018 1120s.

https://accountants-community.intuit.com/articles/1617050-transfer-1120-corp-return-to-1120s-s-corp-...