Hi,
I suggest taking screenshots of your confirmed 48 hour extension and the popup message that you are no longer able to submit. This way, you have proof that you tried to submit on time within your extension window. If you have an academic advisor or class tutor, I would recommend emailing them first and they will be able to direct you to the right people to speak to regarding this. Alternatively, visit your university's student hub or query desk and they will be able to help you in person. It may be a delayed extension error or admin haven't yet updated the due date but this will hopefully be sorted for you.
Good luck!