I also had a client with this issue. They were not  enrolled in coverage through a Health Insurance Marketplace (the had workplace insurance & Medicare)

I chose to follow Lisa's suggestion on form for 1095-A (e.g. enter 0's)

Enter 0's for the Marketplace identifier and Policy number.

Enter $1 for January monthly premium amount and SLCSP (can't enter zeros)

This client had a balance due so doesn't matter if return is pulled for review