Can't Publish New Posts or Upload Images Leads to 404

Hi,

Website is https://www.sixyearslater.com.au with wp files at https://sixyearslater.com.au/pantry/

Nothing is wrong on the front end. Just in the admin section. I can’t seem to add any new posts or insert images into the post. It just leads to a 404 page everytime I click ‘Publish’.

Also tried adding a new post through QuickPress and nothing happened.

Not sure what to do!!