This message is generated when the first argument in Function is not a symbol or a list of symbols.
Since Function has the HoldAll attribute, the value of an argument that evaluates to a symbol or to a list of symbols will not be used unless Evaluate or some similar construction is used to cause that evaluation.
Off[message] switches off the message; On[message] switches it on. For example: Off[Function::flpar].
Examples
Basic Examples(1)
The first argument in Function is not a symbol or a list of symbols:
This shows a valid parameter specification in Function: