Yes, although the extension is not automatic. In HealthCare.gov states, you must apply for the extra time using an Exceptional Circumstances special enrollment period (SEP). You can apply for this SEP by calling the federal marketplace call center at 1-800-318-2596 (not through your healthcare.gov account.) You will have to attest to residing in a FEMA-designated COVID-19 disaster area, which includes the entire U.S. And you will have to attest that you were unable to sign up on time due to the COVID-19 pandemic (for example, if you were hospitalized).
While this extra time to sign up may be available to you, it is still strongly advised that you sign up for 2021 coverage during the regular Open Enrollment period if you can.