New to me CNC toy mill - Taig

What does the error window say?

Usually LinuxCNC will report the exact line in hal where it has a problem

(though with RTAI based version you may need to look at the kernel log (via dmesg) to see the error)

Vinito wrote: pops up the error window instead.

I wish there was a systematic way to work this out.

There is, you need to read what the error window says and address the issues one by one.

Look past the first section of the error listing, you are looking for something like
"ERROR: On line 325 of my-not-very-functional-toy-mill.hal some.pin-that-has-been.spelt-worngly does not exist"

At various points in your HAL file you want the following things