That is why you must use an external power supply (Vcc) that is between 7 to 12 volts to power up the Arduino board and the transistor + relay. The load uses its own power supply, for instance if you ...

As seen in the diagram below, two momentary pushbutton ... do not link Arduino’s ground rail (Gnd) to the relay module’s ground pin (GND). Otherwise, simply connect Arduino 5V and Gnd, and leave VCC ...