Perhaps your original date format is 'DD/MM/YYYY' ? That problem alone would make the formula incorrect...
If your date is already formatted as a proper QlikView date (the easier way to test is to change the number format to Integer and verify that you see 5-digit numbers, starting with 4) - then you can simply format it any way you want:
date(OrderDate, 'YY/MM')
Now, this is still going to be your original date, only presented as a Month. If you wanted to actually replace the date with the Month (so, all 30 days of that month should be replaced with a single value), then you should add the MonthStart() function:
date(MonthStart(OrderDate), 'YY/MM')
If your date was loaded as a string (you don't see integer values when you change the Number format), then you need to cast your string into the proper date value, like this:
Date#(OrderDate, 'DD/MM/YYYY')
After that, you can format the result any way you want, as suggested earlier.
Funny, how the Date#() function tries to recover from an invalid format string. If specifying the year twice is to be considered invalid. What else would you do with something like Date#('16/08/2017', 'YY/MM/YYYY') ?
Fortunately, you can specify many years, but only the last one will be used. For example,
Date(Date#('16/08/2017', 'YY/MM/YYYY')) = Aug 1st, 2017.
