Description
The fieldMapping parameter must use valid field names, field types, and Arcade expressions.
Solution
Check that you have correctly defined the Append Fields and Append Expressions parameters.
For the Append Expressions parameter, the Input Field must be mapped to an Append Field of the same type (that is, text mapped to text, and double mapped to double). Field names must be spelled as they appear in the layer.
Expressions used in the Append Expressions parameter must be valid Arcade expressions. Check that all fields used in the expression are spelled correctly.