Trouble creating shared file between guest and host in Virtual Box (Mojave to Win98)

I am very new to virtual machines and I'm trying to create a shared folder. I created a shared folder and selected both the Auto-mount and Make Permanent options, which lead to a folder I created in my host. When I boot the guest and go to Devices -> Insert Guest Additions CD image, this error pops up:

Unable to insert the virtual optical disk /Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso into the machine Windows 98 2.

Could not mount the media/drive '/Applications/VirtualBox.app/Contents/MacOS/VBoxGuestAdditions.iso' (VERR_PDM_MEDIA_LOCKED).

Result Code: NS_ERROR_FAILURE (0x80004005) Component: ConsoleWrap Interface: IConsole {872da645-4a9b-1727-bee2-5585105b9eed} Callee: IMachine {85632c68-b5bb-4316-a900-5eb28d3413df}

My host is Mac OS Mojave and my guest is Windows 98. I installed the same guest OS on a Windows 10 computer and the same error pops up. I tried reading the instructions for Virtual Machine and reading other questions on this site but I can't seem to find the answer, or I don't understand. Any help would be appreciated, I want to transfer photos to Windows 98.