Secure folder isn't working

Hello. Sorry to hear this. Apart from what you're trying to do, is your secure folder working in every other way?

I'm sorry to hear that you are experiencing this issue. Please can you confirm if you have attempted to move items from your Gallery to the Secure Folder via the following steps?

To add files to your Secure Folder, follow these steps:
Open your Secure Folder and tap the more options icon.
Tap “Add files”.
Choose the type of file you want to add to your Secure Folder and tap on it.
Select the items and tap “Done”.
A pop-up menu will open in your Secure Folder screen and give you the option to "Cancel", "Copy" or "Move". Choose the option you prefer and tap on it.
You can access your files by tapping on "My Files".

https://www.samsung.com/ae/support/mobile-devices/what-is-the-secure-folder-and-how-can-i-use-it/