If I not mistaken got forumer say before they use Platinum Mastercard and having problem with it for the online purchase... So i think that the problem...

Yes,
In maybank2u page  click on the Cards section, on right bottom u see MSOS & have click on it (for your Reference)

U MUST Register as maybank2U User 1st

1) U must go to maybank ATM, main menu select maybank2U Services

2) Select Maybank2U Application

3) Key in your own 6 digit Maybank2u.com PIN (new PIN), NOT your Debit Card PIN

4) Key in same pin to Confirm & for the Secuirity reasons your pin Won't be print on the receipt just blank paper

5) Remember the PIN, used it for 1st time log in www.maybank2u

6) After U have finished register as maybank2u user, go to maybank2u again to register your Debit Card for Online Purchase
  click on Maybank Secure Online Shopping Registration. select "Utilities" funtion

& Fil in your Card no., Password, etc...after that submit it a Successful registration notification will appear once registration is successful & Complete

Now u can use your Debit Card for Online Shopping

it is no longer done so. Not for a year or longer already. Besides, the answer can be found in just the previous page. Do take some time to go through some of the previous posts.
If you have read some of the previous posts before posting, then you would have found our answer already. The answer is there in post #594.

The overseas withdrawal option is strictly for overseas ATM cash withdrawal. If you are not going out of this country or are not going to withdraw for cash when overseas, then it is advisable that you disable this function. Anyway, it is actually disabled by default.
Information in pages between 1-20 might be outdated but not those in the previous few pages. In post #594, blacknwhite0 asked if Maybank debit/credit cards are now automatically enrolled in MSOS and i answered yes. There is also no password to remember anymore unlike last time.

Is your Itunes account a Malaysia account ? Anyway, some Maybankard debit MasterCard Platinum users have reported that they faced problems in some websites. The most recent one was posted here just a few weeks ago and at the end, he changed his to the Cherry VISA debit card.
Any chance the EFTPOS function is disabled ? Try to activate its EFTPOS function again at any CIMB Bank ATM. And do note that the daily purchase limit can only be set at RM1,000, not more and not less.