I am under the minimum 12-hour activation period, waiting for my Secure2u to be activated, and accidentally clicked on the 'Secure2u Activation' when I was performing other tasks on the ATM. Would there be any issue?

Don't worry. The minimum 12-hour cooling period will follow what you have done earlier; it will not be extended.

Once the Secure2u activation period has ended and you can start to approve transactions using Secure2u, you will be notified via SMS, PN, and email.