I have only had this message on two occasions
@zuhaib and both times when I was trying to send two large files to my secure folder and I hadn't waited long enough for the first file to be stored. Moving things to your folder can take a long time especially when the files are large like 4K60 videos. If your phone is telling you this message then it thinks something has not been downloaded in it's entirety and can't store anything else in there till this is done. I would clear the cache on your Secure Folder first and foremost by holding down the app icon in your app tray until a grey banner appears with a small circled 'i' in the top right hand corner. Press this 'i' until a page of options appears. Select storage on this page and a new page comes up with 'clear cache' in the bottom right hand corner. Select this and wait until it shows that your cache is empty. Failing this I would clear the cache partition on your phone. Can I also ask if you can still view items in your secure folder at the moment?.
