Stream

The Answer by Elliott Frisch is correct and robust, and should be accepted.

In addition, for fun, here is a version of that code using streams rather than the conventional for loop. I am not claiming this is better.

I do not know of a way for a stream of an array to affect that array. So instead here I make and return a fresh array.

public static String[] reverse( String[] array ) {
    Objects.requireNonNull( array , "Received null argument where an array of `String` was expected. Message # b5c03336-4b9e-4735-a054-16e43aac059e.") ;
    
    Stream< String > stream = Arrays.stream( array ) ;
    String[] result =
            stream
            .map( ( String s ) -> new StringBuilder( s ).reverse().toString() )
            .toArray(String[]::new) 
    ;
    
    return result ;
}

Usage.

    String arr[] = { "abc" , "def" , "mask" } ;
    String arr2[] = Ideone.reverse( arr ) ;
    
    System.out.println( Arrays.toString( arr ) ) ;
    System.out.println( Arrays.toString( arr2 ) ) ;

See that code run live at IdeOne.com.

[abc, def, mask]

[cba, fed, ksam]

You have your concatenation backwards. Try:

public String reverseString(String str) {
    String res = "";
    for (int i = 0; i < str.length(); i++) {
       res = str.charAt(i) + res;            // Add each char to the *front*
    }
    return res;
}

Note also the simpler, canonical, loop termination condition.

If you want to reverse the array you have stop your iterations in the middle of the array. Otherwise you reverse it twice and therefore it remains the same. Change your for loop to:

for ( int j = 0; j < array.length/2 ; j++){

Unfortunately I do not understand why do you need 2 loops. Just to print the initial array?

This line looks wrong:

for (int j = s[i].length()-1; i >= 0; i--)

It should be:

for (int j = s[i].length()-1; j >= 0; j--)

In other words: the indexes in the inner loop are mistaken, they should be using j instead of i. As a side comment - here's a simpler way to reverse a string:

reverse = new StringBuilder(s[i]).reverse().toString();