Does the switch fail on all 6 power settings, or just one? Have you checked tightness of the screw terminals at the switch?
The Powercraft brand is sold by Aldi - have you tried contacting their Support department? The welder may still be covered by their 3-year warranty.
If these suggestions do not provide a solution, I doubt whether you can get the actual part as a spare. However, from this earlier thread: https://www.mig-welding.co.uk/forum/threads/no-arc-from-new-power-craft-welder.84399/
- it looks like the power selector switch is a fairly simple 1-pole 6-way assembly, so a generic replacement should do the job. Make sure that you know where all the wires go before disconnecting them!
It's often possible to dismantle this type of switch in an attempt to fix loose or burnt contacts, but the usual result of that is a loud ping and a garage floor littered with springs....
Make a note of where all the wires go before you remove it.
Strip it down inside a bucket. Take photos every time you take the next layer apart.
Don't try doing it if you are I a bad mood or short on time. Have a desk light set up so you can see what you are doing.
Odds are you won't be able to replace the burnt bits any way unless it's just a spring that's snapped or you have the patience to make new bits for it.
They are easy enough to get replacements for or if you can work out the wiring you could just replace it with some cheap flick switches. You might even get away with a low, medium and full power if you don't use all the settings anyway