Many are in a category with income low enough they do not pay taxes, however if they have withold from their payroll they would be entitle to a full refund of their witheld taxes. However, these refunds do not become available until after the early months of the following year. For the initial year of enrollment there are no excess funds for tax credits.
A common consumer (patient) may encounter this scenario. Here are some all too common events:
Many Say High Deductibles Make Their Health Law Insurance All but Useless - The New York Times