I'll make sure that you can enter your one-time password (OTP).
Some banks require a special password for account verification and validation of transactions. This is to provide an enhanced level of security on your account. By default, the codes will be sent to your registered mobile number or email.
To isolate the issue, you can log in to your bank's website (OCBC Bank) and make sure that the email or phone number you've entered is correct. If it's not your updated contact information, you'll have to change it. Then, go back to QuickBooks Online and attempt to connect your bank account or download your latest bank transactions to initiate an OTP request again.
However, if the same thing happens, I'd recommend contacting your bank's technical supports. This way, they can look into your account and check you're unable to receive the one-time password.
In addition, here are some articles that you can read to help manage your bank transactions in QuickBooks: