A merchant might decline your RedotPay card for a few reasons:
They don't accept your card's network.
The payment system they use may block the transaction for security reasons.
RedotPay may restrict the transaction to protect your account.
What you can try
Check whether the merchant accepts your card's network.
Add your card to a digital wallet like Apple Pay or Google Pay, and pay through the wallet instead.
Contact support to check whether anything on your account is restricting the payment.