This sounds like ENH-000165385 - "Improve error messaging when making improper configurations to Costs in Network Dataset"
When using a Field Script Evaluator for a Cost in a Network Dataset, if you enter the Fields Alias instead of the Field Name, when you hit OK to close the Network Dataset Properties an error appears "The network dataset was not updated." Instead of providing this nondescript error, providing something that indicates the Field was not entered correctly, or the syntax of the expression was incorrect would be more helpful.