Hello, the error you have is a syntax one, iif() would be something like this eample, using extend.
However this wont solve your main issue, as the line "| where CPU_Used > 85 and CPU_Used < 95" means the next line isnt evaluated, as only the results between 85 --> 95 are shown. Remember that KQL is filtering the input, and you've filtered the answer out, just leaving null at this stage. By default you get "No results found from the last 24 hours" message. Azure Workbooks allow you to supply an alternative message, but I dont know if Azure Dashboards do that?
| extend noresults_ = iif(isempty(CPU_Used),"y","n")