No, there isn’t a universal rule saying you must get a BPO to sell your home. Even so, you may find it helpful to get a BPO when setting the list price, particularly if you plan on selling it on your own versus using a real estate agent.
You usually don’t need to worry about getting a BPO if using a real estate agent, as they’ll often prepare at least a comparative market analysis (CMA) before you agree to list the home with them.