This seems like another way to setup the cluster that said on the post (https://www.techrepublic.com/article/how-to-quickly-install-kubernetes-on-ubuntu/). Or is it using the same steps?
Getting kubectl to run really depends on how you installed it. Basically, if you install and have a proper config file, it should always work. So, either an old file from a previous installation is there or something silly like that (although usually difficult to spot).
Also, make sure the commands don’t fail (some on the post pasted that the step to copy the kubectl config failed). That is the way to authorize to the cluster, so it won’t never work if that step doesn’t work 
If I were you, I’d try removing everything from a previous run and starting from scratch and making sure nothing fails. If it does fail, try to fix that instead of continuing with the next steps. And if you can’t fix it, please report back with the steps you did, why it failed (the error) and what you tried and didn’t work.
This way it will be easier to solve