If you had Medicaid, you should have a Form 1095-B, not 1095-A. Please check your form number, as the information listed on each is different. If you have a 1095-B, the only thing you need to report is that you had insurance for the months listed on the form. A 1095-B does not have a Marketplace Identifier.
The Marketplace Identifier is the insurance company name if you do have a 1095-A..