Most likely, the UM1's power supply and the controller can't handle the addition of a heated bed.
I helped another UM1 owner set up one. You might find some useful information here:
Just to give you an idea. Most beds draw at least 10 amps. At 12V that is 120watts. The UM1's PS is 120watts for everything. And that doesn't consider that the UM1 board will overdrive the bed at 19V. At 19V it will draw about 300watts and 16amps and will probably be well over it's rating.
You should concentrate on getting the temperature reading to work. Do you know what kind of thermistor you got with the kit? The software you need is here - but you have to choose a thermistor:
Once you get the hex file you can use cura to install it.
Also you need to install a 4.7K resistor. Did you do that step?
This post shows a photo of where the resistor goes and how to hookup a typical (but the heat side of things are all different) heated bed:
By the way, the UM typically puts out 19V, not 12V and most people use a separate power supply for the bed, but you don't necessarily have to and one thing at a time.
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