TypeError: type str doesn't define __round__ method

[snippsat]

With f-string and in Python is better to us snake_case PEP-8.

wattage = float(input("What is the wattage of your device?"))
hours_used = float(input("What is the amount of hours used?"))
price_per_kWh = 11.76
electricity_cost = (wattage*hours_used) / (1000*price_per_kWh)
print(f"The cost of electricity is: {electricity_cost:.2f}")