The file couldn’t be saved because you don’t have permission.

Hello Jastu,

We would like to see how we can help you with the permissions on your Mac. Follow the steps in this link to change your permissions: Change permissions for files, folders, or disks on Mac - Apple Support

"Assign permissions to users and groups

1. On your Mac, select a disk, folder, or file, then choose File > Get Info.
2. If the information in Sharing & Permissions isn’t visible, click the arrow . (You may also need to scroll down.)
3. Select a user or group in the Name column, click the pop-up menu, then choose a privilege setting.

• Read & Write: Allows a user to open the item and change it.
• Read only: Allows a user to open the item, but not change its contents.
• Write only (Drop Box): Makes a folder into a drop box. Users can copy items to the drop box, but can’t open it. Only the owner of the drop box can open it.
• No Access: Blocks all access to the item.

You can undo any changes to privilege settings for a user or group in the Sharing & Permissions section since opening the Info window. Before closing the Info window, click the Action pop-up menu  at the bottom of the window, then choose “Revert changes.”"

Cheers!