Help with understanding *errors* as they are shown

General
3

Ok.... But @Steve-Mcl made a simple subflow he posted for me to use on a project.

It works.

I used the flow works as he showed it and I tested it in it's demonstration flow.
It worked.

I stuffed up and didn't use/include a function node upstream in the example he gave me.
So that node not being in the flow I .. used.... it threw an error.
This is his flow.