Eligible for upgrade?

I’m traveling MNL to EWR premium economy in August and was able to upgrade MNL to SIN to business with miles over the phone. They said SIN to EWR had no availability to upgrade but I can keep checking online to see if they do offer it in the future.

On the website and app when I try to upgrade it says “We cannot process your request right now. Please change your selection, or get in touch with your local Singapore Airlines office.”

Does this mean there is no availability or is there a better way to check if it’s possible?