How to properly declare a user variable?

How to properly declare a user variable?

How does one declare and use a non-field variable?

I'm attempting to autoincrement a field, using the example at

https://help.creator.zoho.com/Auto-Increment-Field-Values.html

I receive the error message that the user variable is not a field.
"Field x does not exist in the form "
Yes the variable is not a field, that is the intent of the example cited for the Auto Increment script.

I find no indication in the documentation what form of variable declaration is required.
The information provided at "set variable" is not informative about variable declarations either.

https://help.creator.zoho.com/Set-Variable.html