Hi, I used HSBC card to withdraw cash from ATM in few countries.
You should also have received a 4-digit PIN when you received the 6-digit PIN, they are listed in the same letter sent to you while issuing the Card to you.
Have you ever changed your 4-digit PIN while you used your card at the first time? You should be requested to change the pre-set PIN at the first usage. If not, or you have forgotten it, you need to call HSBC to reset or activate your 4-digit PIN.
Just a reminder, try to use 4-digit PIN to withdraw cash from ATM located inside banks or places more secure becase 4-digit PIN is more risky to get fraud than 6-digit.
I think the default PIN that the bank issues to you is also 6-digit.