Premium tax credits are available to U.S. citizens and lawfully present immigrants who purchase coverage in the Marketplace and who have income between 100% and 400% of the federal poverty level. Premium tax credits are also available to lawfully residing immigrants with incomes below 100 percent of the poverty line who are not eligible for Medicaid because of their immigration status. (Generally, immigrants must lawfully reside in the U.S. for five years before they can become eligible for Medicaid.)
In addition, to be eligible for the premium tax credits, individuals must not be eligible for public coverage—including Medicaid, the Children's Health Insurance Program, Medicare, or military coverage—and must not have access to health insurance through an employer. (There is an exception in cases when the employer plan is unaffordable because the employee share of the premium exceeds 9.78% of the employee's income in 2020. 
There is also an exception in cases where the employer plan doesn’t provide a minimum level of coverage.)
Source: KFF
