Raspberry Pi 4B giving 0.3V output when GPIO is set to True or High

Question

I have a Raspberry Pi 4B, 4GB version. When I try to run the following code it gives 0.3V output. I mean it works, on-0.3V, off-0V. But I need 3.3V output. I measured it with a voltmeter, as the interested device was not working getting only 0.3V from Pi.

import time
GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)
GPIO.setup(21,GPIO.OUT)
print ("LED on")
GPIO.output(21,True)
time.sleep(10)
print ("LED off")
GPIO.output(21,False)

Answer 1

As others have pointed out in Comments (in response to your comment - which should be edited into your Question) the GPIO can only source a maximum of 16 mA and may be damaged.

You need a transistor or similar to control higher currents. There are hundreds of Answers on this site with recommended circuits (depending on the specifications of the pump).

Answer 2

GPIO have extremely limited current output; when overloaded, the voltage drops to near zero. This is a clear indication that you have "screwed up".

Use a transistor (or amplifier) to drive your "tiny dc pump" as shown in the schematic below:

^{simulate this circuit – Schematic created using CircuitLab}