The final step a a years worth of testing on the first core SLS booster is supposed to be a full duration static fire, running the engines in a profile mimicking that of a real flight. This test was supposed to be 8 minutes long but unfortunately the test terminated early for technical reasons.

Right now the information we have is not particularly revealing so we don't know whether this is going to push the first flight of the core back far enough to make it miss the 2021 schedule.