the process is closed but in red letters it states that they are accepting back up nominations. It is regarding one position that was opened but now it is closed, it is hard to me to understand what they mean by saying this ?
Noun backup (plural backups)
A reserve or substitute. If the goalkeeper is injured, we have a backup.
In this context, it means that they have selected someone who they want for the position, but are worried that the person might not accept or be otherwise unavailable. In case this happens, they want a backup person ready to step into the position instead of the original candidate that they would prefer.
They want to let you know that you are now applying to be that backup. If the original candidate accepts the position, it doesn't matter how well your resume looks or how well you interview for the position; the position is effectively filled. Only in case of something going wrong will it be possible for you to get the position.