The service animal should still be allowed in.
The employer should work with you to adjust your schedule so that you can finish in time and still meet the needs of the job. An employer must try to find a reasonable change to policies, settings, schedules, and so on, that would enable you to get the job done.
An important word to remember here is “reasonable.” The law does not require someone or some place to make changes that would cost so much that it would be a “hardship,” meaning that it would be too costly or difficult for them based on their ability to do it.
If the store with steps is a small, local store with a narrow sidewalk, it might be too difficult for them to install a ramp. However, the store would still need to explore other ways to provide you access.