What is "Effects" in Program Graph?
What you wrote is correct. The actions $\textit{coin}$ and $\textit{ret_coin}$ do not change the values of the variables $\textit{Var} = \{\textit{nsoda}, \textit{nbeer}\}$, i.e., the number of soda and beer cans in the machine.

Inserting a coin is only possible in the $\textit{start}$ location, and with this action the location changes to $\textit{select}$. If you’re wondering how the machine keeps track of payment at all, this is it: By the PG's two locations.
Only after the user selects a beverage, a can is dispensed, and the variable evaluation changes (the effect of $\textit{[sb]get}$).

Similarly, returning the coins (which can only happen in the case the vending machine has neither beer nor soda cans) does not change the number of cans.

You could propose a different model that does track the exact amount of coins inserted as a variable, and define


$\textit{Effect}(\textit{coin},η) = η[\textit{coins_inserted} := \textit{coins_inserted} + 1]$ as well as
$\textit{Effect}(\textit{ret_coin},η) = η[\textit{coins_inserted} := 0]$


but the example given does not do so.