Bottom line up front - in your case, just forget about these 'declined' entries and use the card with NFC directly on gates.

Explanation: top-up of Ezlink and Nets FlashPay cards - the stored value cards for the Singapore transit system - can be done only using Singapore-issued cards. That is what the sign says. It does not apply to using foreign-issued cards to tap on entrance-exit gates.

Using overseas-issued cards with entrance/exit gates is supported with both a mobile wallet - Apple/Google/Samsung pay and using cards directly tapping on gates (assuming that cards have NFC support). Using cards directly is faster and safer - there are problems, especially with Google pay and overseas-issued cards, but there was no problem when tapping the card directly.