Yes, you can take out a life insurance policy on your step-parent if you would suffer a financial loss i he were to die.
There does exist an insurable interest between parents and their children, siblings, spouses, and business partners.
Since you may lose the financial support your step-parent provides for you if he/she died, you would probably be able to purchase a life insurance policy.
However, it may depend on your age, and your current relationship with your step father. For instance, does he/she currently provide financial support for your living expenses, etc.?
Here's how you can learn more about What is Insurable Interest, and compare life insurance rates and plans from leading life insurance companies.