The DC jack voltage has to be between 7V to 12V for the UNO on board regulator to work.
If you put 5V into the DC jack, you will have problems getting the UNO working.
If your external power supply is regulated 5 volts, you can connect it to the 5V and GND pins on the Arduino as in my image.
Remove these connections when powering the Arduino from the USB connector.
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