Mostly, Stop Safely Now warning shows up on Ford's hybrid vehicles when there is a malfunction in the electric powertrain. The error codes needs to be checked with OBD2 scanner to know the exact cause.
Its a common issue with Ford's hybrid vehicle, and considering the complicated setup of hybrids, all you can do is take it to the Ford dealer. They may also have a hard time diagnosing this issue if they don't have an experienced mechanic for hybrids on site.