Your problem is caused by calling the computeTax method using a string (state) as the first argument and a double (sales) as the second argument.
Your method computeTax Method expects a double as the first argument (sales) and a string (sate) as the second argument.
Either change the calling code to
tax = computeTax(sales, state);
or change the computeTax method signature to
public static double computeTax(String state, double sales)