Your firmware has set a limit of 270 °C, normally, (default Marlin configured value) this is 275 °C. It appears that the Anet E10 developers have edited the value if you cannot exceed the 270 °C setpoint.
The configuration file for Marlin firmware has the following maximum temperature limit set for the first hotend:
#define HEATER_0_MAXTEMP 275
You can ...