Type error [M0073], expected mutable assignment target

Developers
1 
    private stable var adIdCount : Nat = 0;
   public func setAdidCount(x:Nat):async (){
    adIdCount:=x;
 }

//type error [M0073], expected mutable assignment target

I’m a little confused

2

dfx deploy
//type error [M0073], expected mutable assignment target

3

Your example compiles fine for me: Motoko Playground - DFINITY

Are you sure the part you showed is what caused the error you see?