The error message you provided doesn't appear to pertain to the screenshot/formula. "fieldName" is not present in the formula. I'm guessing that the output of "Business Unit Ref ID" is being added in "fieldName" elsewhere. What does that formula look like?
In your response above to dcornwell, you mention a new error: "Error calculating input for field 'BusinessUnitRefID': no method 'where' for true". This is interesting because it appears that the value for "Custom fields" is expected to be an array of objects, but is arriving as a boolean (and you can't .where a boolean).
Without knowing more, I could only classify this as "very odd" and I recommend looking at the input data first. If the input data is arriving in an unexpected format, that may be the root cause of the formula backfiring. Once you confirm the input data is correct, read on for a suggestion on fixing the issue.
When any of the where methods return no results, you'll hit an error. This is because the other methods used to transform the result (upcase, '+', split, gsub) are string methods that will not work on nil data types.
If you really want to retain the formula as is without rewriting it into a Ruby snippet or multiple actions, I recommend first checking for the value before transforming it like this:
## test to see if a result is present without transforming
(([Custom Fields].where("Name": "xxx").pluck('Value').first.present? :
## now transform and test each case in a callback hell of nested ternary operators
(([Custom Fields].where("Name": "xxx").pluck('Value').first.upcase.include?("xxx") ?
"result when true" :
## next ternary operator here
{test condition} ? {true result} :
## next ternary operator here
{test condition} ? {true result} :
## ... and so on until the end of time
Edit: I should also mention that if the .where("Name": "xxx") is always testing for the same name "xxx", you can check for presence once at the beginning of the formula. However, if the formula checks for multiple names throughout the formula (I can't tell, they are blurred), then you'll need to test for presence each time (for each name) at the start of a new nested ternary operator. I'm getting dizzy just thinking about it!
