When an automation encounters an input with a missing value or an input that is incorrectly linked, an error alerts you that this input has an invalid, or missing, value.
The most common example of this is when inputs are established outside of a Symbol Loop or Position Loop. Each loop supplies a default input for its position or symbol, which is intended to be used throughout the automation when referencing symbol or position, so the two are connected.
If a symbol or position input is used in place of the Loops default action input when the automation processes, it encounters a position or symbol that does not stem from the loop and can not validate.
In other words, the position or symbol it is referencing is not valid because it is not married to the loop.
When building automations that utilize a position or symbol loop, it's important to remember to establish your Loop before the other decisions and then link any symbol or position value to the default action that is supplied with the loop.