If the answer is yes to both questions, you need to talk to your 401(k) administrator and the preparer of the Schedule K-1 if you want to be able to make this elective deferral.
Note that the election to make the elective deferral for 2018 was required to have been made by December 31, 2018, so it odd that the elective deferral is not reflected in the code R amount if things were done properly.
