lme4 nlme – How to use a Linear Mixed Models (LMMs) to compare a fixed factor with 3 levels, which differ in size

I want to run an LMM with a 3-level fixed factor “block_condition” (levels: A,B,C) and 2-level block_number (1,2). SubjectID was also added as a random factor to the model.

Now the problem is that I have for block_condition A and B – two repetitions/levels (A1, A2, and B1, B2), but in condition C, I have only 1 repetition/level (C1), and I don’t know how to run an LMMs with one condition that has level less when I try to set the following LMM:

lmer(DV1 ~ block_number * block_condition + (1|sub), data = dfT1).

Is there to run such LMM? or a legitimate way to complete/replicate the missing information in condition C – for the sake of the specific comparison?

Thanks in advance,
G