Ah, ok. That’s your external sd card. You need to request access for Magic Dosbox in Global settings. Android blocks access for god knows what reasons.
Open Magic Dosbox
Tap Settings icon (the cogwheels)
Tap Global settings
Scroll down to sd card permissions, tap Request permission
Select the root of your external sd card from the Android file manager
Maybe you’ll have to tap menu button – show external sd card inside the file manager, of it’s hidden.