If you are clicking a button to instigate this code, then "Target" means nothing and that is (at least one reason) why your code is failing.
I'm not entirely clear what you want but the following code should do this:
If an "a" is found in either or both named ranges, both named ranges are cleared.
If an is not found in either named range then they are both populated with "a"s
If that is not what you want, please add more detail.
Dim aFound As Range
With Union(Range("Named_Range_1"), Range("Named_Range_2"))
Set aFound = .Find(What:="a", LookIn:=xlValues, _
LookAt:=xlWhole, MatchCase:=True, SearchFormat:=False)
If aFound Is Nothing Then
.Value = "a"